Modle 6 : Preventive, Emergency and Restorative Control
Lecture 29 : Emergency Control : An example
Objectives
In this lecture you will learn the following
An example to illustrate the system angular instability and islanding
A simple 2 machine example
Consider the two machine system shown below:
Suppose it is subject to a fault on one of the interconnecting lines.
This fault is cleared by tripping the lines using Circuit Breakers which are triggered by
protective relays
, What are the possible consequences of such a disturbance ?
Possible Consequences
The possible consequences of a large disturbance like a fault (followed by line clearing) can be :
a) The system settles to a new acceptable equlibrium after some initial transients die down.
The system settles to a new equilibrium, but the equilibrium is violative of some steady state
b) equipment limit (leading to tripping out of that equipment).
c) The system does not attain a new equilibrium due to angular or voltage instability.
Voltage instability leads to unacceptably low voltages (which can be corrected by under-voltage load
shedding), while angular instability (loss of synchronism) leads to violent excursions in current, voltage and
power leading to equipment damage. Therefore, the generators which have lost synchronism have to be
disconnected from each other. This may occur "naturally" due to distance relays (which mistake the large
variations in voltage and current for a fault and trip the interconnecting lines), or intentionally -- controlled
system separation --- by action of specially designed out of step relaying schemes. This situation is shown
below.
The resulting two islands may have substantial real and reactive power deficit or surplus. This may result in
decline or rise of frequency and//or voltage. Therefore excitation control, governors, generator overspeed
control, and under-frequency/under-voltage load shedding are essential to make the islands stable.
We now anaylse this disturbance ...
In order to understand the various possibilities, we consider the system shown below which is modelled as
follows:
A generator in this example is modelled as a voltage source (constant magnitude) behind a reactance.
The angle dynamics is described by the swing equation:
Lecture 29 : Emergency Control : An example
Objectives
In this lecture you will learn the following
An example to illustrate the system angular instability and islanding
A simple 2 machine example
Consider the two machine system shown below:
Suppose it is subject to a fault on one of the interconnecting lines.
This fault is cleared by tripping the lines using Circuit Breakers which are triggered by
protective relays
, What are the possible consequences of such a disturbance ?
Possible Consequences
The possible consequences of a large disturbance like a fault (followed by line clearing) can be :
a) The system settles to a new acceptable equlibrium after some initial transients die down.
The system settles to a new equilibrium, but the equilibrium is violative of some steady state
b) equipment limit (leading to tripping out of that equipment).
c) The system does not attain a new equilibrium due to angular or voltage instability.
Voltage instability leads to unacceptably low voltages (which can be corrected by under-voltage load
shedding), while angular instability (loss of synchronism) leads to violent excursions in current, voltage and
power leading to equipment damage. Therefore, the generators which have lost synchronism have to be
disconnected from each other. This may occur "naturally" due to distance relays (which mistake the large
variations in voltage and current for a fault and trip the interconnecting lines), or intentionally -- controlled
system separation --- by action of specially designed out of step relaying schemes. This situation is shown
below.
The resulting two islands may have substantial real and reactive power deficit or surplus. This may result in
decline or rise of frequency and//or voltage. Therefore excitation control, governors, generator overspeed
control, and under-frequency/under-voltage load shedding are essential to make the islands stable.
We now anaylse this disturbance ...
In order to understand the various possibilities, we consider the system shown below which is modelled as
follows:
A generator in this example is modelled as a voltage source (constant magnitude) behind a reactance.
The angle dynamics is described by the swing equation:
