Module 4 : Voltage and Power Flow Control
Lecture 19 : Power Flow Control
Objectives
In this lecture you will learn the following
Power transfer capability of a network may be limited due inability to control power flow in transmission
paths.
What are the ways by which one may control real power flows?
An important feature of interconnected AC systems is that power flow control through specific control paths is
often quite difficult. This is because power flows in these paths as per Kirchhoff laws: the line parameters,
topology of the network, generation and load location determine the value of flows. The power flows are not
directly dependent on transmission line ownership, contracts, thermal limits or losses!
Inability to control power flows may result in situations wherein, some paths may be lightly loaded and some
overloaded (to understand what one means by loadability see the discussion on line loadability in module 2).
The figure given on the right
demonstrates a possible
scenario wherein there is power
flow in interconnecting tie lines
of the two areas although each
of them are individually self-
sufficient. Thus the tie lines are
unnecessarily loaded. This may
be a problem if the capacity of
the tie lines is limited and one
actually wishes to transfer
power from one area to
another. This is because a part
of the line capacities are utilized
due to the undesired "loop
flow".
Tie line power flow control by AGC was discussed in the previous module. While cumulative power exchange
between the two areas through both tie lines may be controlled by adjusting the generated power in areas A and
B, independent power flow control in each individual line connecting the 2 areas is not possible by generation
control alone. In the above figure, cumulative power flow in the two tie lines is zero. It can be changed by
reducing generated power in one area and increasing in another. However, the individual power in each line
cannot be controlled in this fashion and depends on line parameters and Kirchoff's laws.
Therefore, special measures have to be taken in order to control power flow through individual transmission
paths. Some of these are considered in the following lectures.
Power Flow control can be achieved by changing effective line parameters by connection of lumped series
capacitors.
Series Compensation of lines
This involves changing the effective series reactance of lines by connecting capacitors in series with a line.
Reducing line reactance also improves stability of a system (see module 2 for a discussion of large disturbance
stability).
In the figure given below, the power flow in the branch with reactance 2x can be increased by compensating it
, with a series capacitor.
Since the amount of loading changes with time, the
amount of series compensation may be varied by
bypassing (shorting out) these capacitors when not
necessary.
Normally a capacitor can allow for short duration over-
rating for a few seconds. This allows insertion of larger
capacitive reactance into a line for a short time in order
to improve angular stability.
Alternatively, they may be controlled using power
electronic controllers. For example, a TCR (discussed in
the section on voltage control), may be connected in
parallel to the series capacitor and its effective
reactance may be controlled by controlling the firing
angle delay. This device is called a Thyristor
Controlled Series Compensator (TCSC).
Thyristor Controlled Series Compensator
The effective reactance is the
parallel combination of a
fixed capacitor and variable
reactance.
Parallel resonance may occur
at certain values of firing
angle. The characteristics of
a TCSC are given on the
right.
Why cannot we increase the
effective reactance of the
parallel combination to much
larger values say upto the
parallel resonance point?
One of the major consideration of any series connected device is the need to bypass the device when faults occur
nearby. Faults cause large overcurrent in a line, which in turn results in overvoltage across a capacitor. Therefore,
Metal Oxide Varistors (which are nonlinear devices which have very low resistance when voltage magnitude is greater
than a particular value) and circuit breakers are generally connected across series connected devices. Note that during
Lecture 19 : Power Flow Control
Objectives
In this lecture you will learn the following
Power transfer capability of a network may be limited due inability to control power flow in transmission
paths.
What are the ways by which one may control real power flows?
An important feature of interconnected AC systems is that power flow control through specific control paths is
often quite difficult. This is because power flows in these paths as per Kirchhoff laws: the line parameters,
topology of the network, generation and load location determine the value of flows. The power flows are not
directly dependent on transmission line ownership, contracts, thermal limits or losses!
Inability to control power flows may result in situations wherein, some paths may be lightly loaded and some
overloaded (to understand what one means by loadability see the discussion on line loadability in module 2).
The figure given on the right
demonstrates a possible
scenario wherein there is power
flow in interconnecting tie lines
of the two areas although each
of them are individually self-
sufficient. Thus the tie lines are
unnecessarily loaded. This may
be a problem if the capacity of
the tie lines is limited and one
actually wishes to transfer
power from one area to
another. This is because a part
of the line capacities are utilized
due to the undesired "loop
flow".
Tie line power flow control by AGC was discussed in the previous module. While cumulative power exchange
between the two areas through both tie lines may be controlled by adjusting the generated power in areas A and
B, independent power flow control in each individual line connecting the 2 areas is not possible by generation
control alone. In the above figure, cumulative power flow in the two tie lines is zero. It can be changed by
reducing generated power in one area and increasing in another. However, the individual power in each line
cannot be controlled in this fashion and depends on line parameters and Kirchoff's laws.
Therefore, special measures have to be taken in order to control power flow through individual transmission
paths. Some of these are considered in the following lectures.
Power Flow control can be achieved by changing effective line parameters by connection of lumped series
capacitors.
Series Compensation of lines
This involves changing the effective series reactance of lines by connecting capacitors in series with a line.
Reducing line reactance also improves stability of a system (see module 2 for a discussion of large disturbance
stability).
In the figure given below, the power flow in the branch with reactance 2x can be increased by compensating it
, with a series capacitor.
Since the amount of loading changes with time, the
amount of series compensation may be varied by
bypassing (shorting out) these capacitors when not
necessary.
Normally a capacitor can allow for short duration over-
rating for a few seconds. This allows insertion of larger
capacitive reactance into a line for a short time in order
to improve angular stability.
Alternatively, they may be controlled using power
electronic controllers. For example, a TCR (discussed in
the section on voltage control), may be connected in
parallel to the series capacitor and its effective
reactance may be controlled by controlling the firing
angle delay. This device is called a Thyristor
Controlled Series Compensator (TCSC).
Thyristor Controlled Series Compensator
The effective reactance is the
parallel combination of a
fixed capacitor and variable
reactance.
Parallel resonance may occur
at certain values of firing
angle. The characteristics of
a TCSC are given on the
right.
Why cannot we increase the
effective reactance of the
parallel combination to much
larger values say upto the
parallel resonance point?
One of the major consideration of any series connected device is the need to bypass the device when faults occur
nearby. Faults cause large overcurrent in a line, which in turn results in overvoltage across a capacitor. Therefore,
Metal Oxide Varistors (which are nonlinear devices which have very low resistance when voltage magnitude is greater
than a particular value) and circuit breakers are generally connected across series connected devices. Note that during
