08-11-2012, 03:34 PM
Preventive, Emergency and Restorative Control
[attachment=38592]
Transition from an alert state to an emergency state
If a system operator infers from the operating data that a system is in an alert state, then he takes preventive
control actions to bring the system back to a normal state. However, it is possible that the system operator is
unable to act in time before a contingency actually occurs. A grid may even operate insecurely (in an alert
state) due to a high cost of preventive control or due to inadequate reserve margins. However this situation is
undesirable since it may lead to blackouts (if emergency control actions fail) which can cause great economic
loss. The classification of a system state as a normal or alert state is based on simulating some disturbances.
Often, even though the system has been classified as being in a normal state, several improbable disturbances,
which would not have been analyzed for doing this classification, take place.
Therefore the system can transit from a perceived alert state to an emergency state if no preventive controls
are exercised and a contingency occurs, or may directly transit to an emergency state from a perceived normal
state if an unanticipated sequence of several contingencies occur. If the system does go into an emergency
state some equipment limits are exceeded which may cause further tripping of equipment, thereby worsening
the situation and may cause a complete blackout.Emergency control actions (manual or automatic) are required
to retrieve the situation. If there is a thermal overload of an equipment then there is some time to act and
quick "heroic action" from a system operator would be needed. However in most cases one has to rely on
automatic controls to quickly respond to such a situation.
Other Emergencies
For simplicity, we have restricted our discussion of alert and emergency states arising from line thermal
overload. However, it should be recognised that many disturbances may lead to other equipment limits being
violated.
For example, a sudden load tripping may cause overvoltages in long EHV lines especially if the transmission
line loading goes significantly below the Surge Impedance Loading. This may cause line tripping on
overvoltages. Emergency control can be in the form of insertion of shunt reactive power absorbing devices
(see Module 4 for the various voltage control devices in a system).
Angular Instability and Emergency Control
Let us now consider a situation wherein an emergency is caused by loss of synchronism between generators.
Consider a four generator system shown (below/right/left). We shall assume that the loads at the 2 buses shown
in the figure are not voltage dependent and that losses in the system are neglible. Suppose that a fault occurs
(e.g., a short circuit between phase to ground) on one line which is carrying 500 MW) This is detected by relays
at both ends of the line and they send a trip signal to the circuit breakers which disconnect this line. This is
usually done in a very short time (about 100 ms). The loss of this line causes power to get diverted to the
parallel line.
Angular Instability and Emergency Control : Transient Phenomena
While the steady state scenario following loss of line does not cause violation of equipment limits, the question
arises: Will the system reach a steady state ?
The worrisome aspect of this disturbance is that the transient behaviour of the relative generator speeds after
the disturbance may not be stable. During the fault, generator speeds deviate due to a sudden change in the
electrical power due to the fault. After the line is tripped, the generator speeds do not directly go into steady
state because of the deviation caused due to a fault. However, if the relative angular differences between the
machines are not too large, then the electrical torques try to pull all the generator speeds together in
synchronism, i.e., the system is angular stable (for a detailed discussion on angular stability, refer to Module 2).
The typical waveforms of generator speeds, power flow (P) and the phase angular difference between the bus
voltages at both ends of the line shown below. The power oscillates and eventually settles at a constant value
after some time (damper windings in generators can contribute to damping these oscillations). Also, all
generator speeds reach the same value.
[attachment=38592]
Transition from an alert state to an emergency state
If a system operator infers from the operating data that a system is in an alert state, then he takes preventive
control actions to bring the system back to a normal state. However, it is possible that the system operator is
unable to act in time before a contingency actually occurs. A grid may even operate insecurely (in an alert
state) due to a high cost of preventive control or due to inadequate reserve margins. However this situation is
undesirable since it may lead to blackouts (if emergency control actions fail) which can cause great economic
loss. The classification of a system state as a normal or alert state is based on simulating some disturbances.
Often, even though the system has been classified as being in a normal state, several improbable disturbances,
which would not have been analyzed for doing this classification, take place.
Therefore the system can transit from a perceived alert state to an emergency state if no preventive controls
are exercised and a contingency occurs, or may directly transit to an emergency state from a perceived normal
state if an unanticipated sequence of several contingencies occur. If the system does go into an emergency
state some equipment limits are exceeded which may cause further tripping of equipment, thereby worsening
the situation and may cause a complete blackout.Emergency control actions (manual or automatic) are required
to retrieve the situation. If there is a thermal overload of an equipment then there is some time to act and
quick "heroic action" from a system operator would be needed. However in most cases one has to rely on
automatic controls to quickly respond to such a situation.
Other Emergencies
For simplicity, we have restricted our discussion of alert and emergency states arising from line thermal
overload. However, it should be recognised that many disturbances may lead to other equipment limits being
violated.
For example, a sudden load tripping may cause overvoltages in long EHV lines especially if the transmission
line loading goes significantly below the Surge Impedance Loading. This may cause line tripping on
overvoltages. Emergency control can be in the form of insertion of shunt reactive power absorbing devices
(see Module 4 for the various voltage control devices in a system).
Angular Instability and Emergency Control
Let us now consider a situation wherein an emergency is caused by loss of synchronism between generators.
Consider a four generator system shown (below/right/left). We shall assume that the loads at the 2 buses shown
in the figure are not voltage dependent and that losses in the system are neglible. Suppose that a fault occurs
(e.g., a short circuit between phase to ground) on one line which is carrying 500 MW) This is detected by relays
at both ends of the line and they send a trip signal to the circuit breakers which disconnect this line. This is
usually done in a very short time (about 100 ms). The loss of this line causes power to get diverted to the
parallel line.
Angular Instability and Emergency Control : Transient Phenomena
While the steady state scenario following loss of line does not cause violation of equipment limits, the question
arises: Will the system reach a steady state ?
The worrisome aspect of this disturbance is that the transient behaviour of the relative generator speeds after
the disturbance may not be stable. During the fault, generator speeds deviate due to a sudden change in the
electrical power due to the fault. After the line is tripped, the generator speeds do not directly go into steady
state because of the deviation caused due to a fault. However, if the relative angular differences between the
machines are not too large, then the electrical torques try to pull all the generator speeds together in
synchronism, i.e., the system is angular stable (for a detailed discussion on angular stability, refer to Module 2).
The typical waveforms of generator speeds, power flow (P) and the phase angular difference between the bus
voltages at both ends of the line shown below. The power oscillates and eventually settles at a constant value
after some time (damper windings in generators can contribute to damping these oscillations). Also, all
generator speeds reach the same value.