Voltage Collapse
And Sympathy Trips
Objectives
• Identify the symptoms an electric system displays
preceding a Voltage Collapse.
• Identify the 2 types of System Load and how each is
affected by Low Voltage.
• Recognize the definition of Dynamic Instability.
• Identify the definition of Surge Impedance Loading (SIL)
of a transmission line and the effects of a line loaded
above or below its SIL.
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2
Objectives
• Define Voltage Stability
• Define Voltage Collapse
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3
Recognizing a Voltage Collapse
IEEE Definitions:
• Voltage Stability
– is the ability of a system to maintain voltage so that when load is
increased, power will increase, so that both power and voltage
are controllable.
• Voltage Collapse
– is the condition by which voltage instability leads to loss of
voltage in a significant part of the system.
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4
Recognizing a Voltage Collapse
IEEE Definitions:
• Voltage Security
– is the ability of a system, not only to operate stably, but also to
remain stable (as far as the maintenance of system voltage is
concerned) following any reasonably credible contingency or
adverse system change.
– A system enters a state of voltage instability when a disturbance,
increase in load, or system changes causes voltage to drop
quickly or drift downward, and operators and automatic system
controls fail to halt the decay. The voltage decay may take just a
few seconds or ten to twenty minutes. If the decay continues
unabated, steady-state angular instability or voltage collapse will
occur.
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5
Recognizing a Voltage Collapse
Surge Impedance Loading
The SIL is the loading in MW at which the MVAR from the natural
capacitance of a line exactly cancels the MVAR the line needs to support its
voltage.
At Surge Impedance Loading
Source
Load
MW
MVAR
MW
MVAR
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6
Recognizing a Voltage Collapse
Surge Impedance Loading
The SIL is the loading in MW at which the MVAR from the natural
capacitance of a line exactly cancels the MVAR the line needs to support its
voltage.
Below Surge Impedance Loading
Source
Load
MW
MW
MVAR
MVAR
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7
Recognizing a Voltage Collapse
Surge Impedance Loading
The SIL is the loading in MW at which the MVAR from the natural
capacitance of a line exactly cancels the MVAR the line needs to support its
voltage.
Above Surge Impedance Loading
Source
Load
MW
MVAR
MW
MVAR
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8
Recognizing a Voltage Collapse
• Loss of Synchronization:
The point at which the generating unit loses its electrical bond with
the system and begins to over speed and “slip poles”.
• Active (MW)
Active power is what does the work in the system.
• Resistive Loads (MW)
During a voltage dip resistive load current will decrease.
• Inductive (Motor) Loads (MW & MVAR)
During a voltage dip motor load current will increase - the lower the voltage,
the more current they draw.
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9
Recognizing a Voltage Collapse
• Reactive (MVAR)
– MVARs are produced by capacitive loads like transmission lines
capacitor banks and generators.
– MVARs are absorbed by inductive loads, like transformers,
reactors, motors and generators.
– MVARs are needed to support voltage and produce magnetic
fields in the AC system.
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Generator Excitation Systems
• The excitation system maintains the desired terminal
voltage by means of automatic controls that measure the
terminal voltage and compare it to the preset (“desired”)
value.
• The AVR is the device that controls the “set point”
automatically to the excitation system.
• When the AVR is in manual mode the set point will not
be controlled and will not respond to disturbances that
require voltage support.
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Generator Excitation Systems
• As voltage rises above the set point the generator
excitation system begins to lower the excitation current
and consume MVARs.
• The generator will continue to absorb MVARs until it
reaches its Under Excitation Limit and at that point will
trip to prevent the generator from overheating and
damaging rotor windings.
• At this point the generator is dangerously close to
“slipping a pole” if there is a large voltage fluctuation on
the transmission system.
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Generator Excitation Systems
• As voltage drops below the set point the generator
excitation system begins to raise the excitation current
and provide MVARs.
• The generator will continue to produce MVARs until it
reaches its Over Excitation Limit and will trip to prevent
damage to the generator.
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Generator Excitation Systems
• The connection strength of the generator to the
transmission system is determined by the strength
of the excitation system field on the generator.
– The further the generator goes into the lead the weaker the
connection becomes.
• In order to understand the relationship between the
generator and transmission we must discuss power
angle.
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Power Angle Review
δ
V1
V2
Sending BUS
Receiving Bus
XL
Power Angle
Bus 1
P= V1 V2 sinδ
XL
Bus 2
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15
Recognizing a Voltage Collapse
1 - Remote Generation
Gen Station A
Load Center
And Gen
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16
Recognizing a Voltage Collapse
2 - Local Load center
Gen Station A
Load Center
And Gen
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Recognizing a Voltage Collapse
3 - 345KV Transmission Lines
Gen Station A
Load Center
And Gen
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Recognizing a Voltage Collapse
Gen Station A
Maximum Flow
F
L
O
W
Load Center
And Gen
Current Flow
0 30
90
180 Power Angle
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19
Recognizing a Voltage Collapse
Gen Station A
F
L
O
W
Maximum Flow
Current Flow
0
60 90
Load Center
180 Power Angle
20
20
Recognizing a Voltage Collapse
Machine Dynamic Stability
• During steady state operation the turbine throttle valve is adjusted
so that the mechanical power is equal to the mechanical load
applied to the generator.
• During the dynamic period these two quantities will not be equal and
the turbine-generator rotor will over speed.
• The change in electrical output results in an immediate change in
the mechanical load applied by the generator to the turbinegenerator rotor assembly. This results in an imbalance between the
mechanical power from the turbine and the mechanical load applied
by the generator.
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Machine Dynamic Stability
• Since the generator electric angle is related to the position of the
rotor. Any change in the rotor speed (due to the imbalance) will, in
turn, result in changes in the generator output. There will be an
interaction between the system and rotor speed.
• The term “equal area criteria” is used to describe the comparison of
the area that represents the energy stored in the turbine generator
during the acceleration period and the area that represents the
energy removed from the turbine generator in the deceleration
period.
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Recognizing a Voltage Collapse
If one line trips: The power angle drops from the initial operating point (A) to point (B).
This is because, for an instant, the phase angle is at its original value of 30 deg. The
electrical power is now less than the mechanical power.
Power – Angle Curve Trip of One Line
MW Output – System 1 Generator
Turbine Mechanical
Power (Input)
A
1000 MW
With 3 Lines
(Pre-Disturbance)
B
With 1 Line
(Post-Disturbance)
Angle in degrees
30
60
90
120
23
150
180
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Recognizing a Voltage Collapse
The turbine – generator accelerates. The acceleration advances the generator rotor
phase angle increasing the δ between busses. As the angle increases from 30 to
about 50 the power output increases from point (B) to point (C).
Power – Angle Curve Trip of One Line
MW Output – System 1 Generator
Turbine Mechanical
Power (Input)
A
1000 MW
F
With 3 Lines
(Pre-Disturbance)
C
B
With 1 Line
(Post-Disturbance)
Angle in degrees
30
60
90
120
24
150
180
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Recognizing a Voltage Collapse
The turbine –generator begins to decelerate from (C) to (D). Once the unit reaches
(D) it has increased the phase angle to approx. 60 (E) and continues to output the
MW value from pre disturbance.
Power – Angle Curve Trip of One Line
MW Output – System 1 Generator
D
Turbine Mechanical
Power (Input)
A
1000 MW
F
E
With 3 Lines
(Pre-Disturbance)
C
B
With 2 Line
(Post-Disturbance)
Angle in degrees
30
60
90
120
25
150
180
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Recognizing a Voltage Collapse
• If the system is operating with a δ close to 90 deg. and we attempt to
transmit more power across the line by putting more power into the
turbine on the sending generator the phase angle will increase beyond
90 deg. and the power transfer would decrease.
• If the phase angle exceeds 90 deg. the rotors on the sending end
generators will begin accelerating and the rotors on the receiving end
generators will decelerate.
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26
Recognizing a Voltage Collapse
Gen Station A
F
L
O
W
Maximum Flow
Current Flow
0
90
Load Center
180 Power Angle
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Recognizing a Voltage Collapse
Transmission Tripping
The transmission line trips at about 60 degrees. The power angle drops to (B).
The turbine is now at 60 Hz and the power angle is at 70 degrees. The turbine continues to decelerate
to maintain speed.
The turbine reaches maximum deceleration.
MW Output – System 1 Generator
At (D) the turbine is accelerating again and
now has a power angle of close to 120
degrees.
C
A
D
Turbine Mechanical
Power (Input)
B
With 3 Lines
(Pre-Disturbance)
The area inside (A), (B) and (C) is
larger than the area inside (C) and (D)
and is therefore outside of the “equal
area criteria” rule and this turbine will
go out of step with the system if not
tripped.
The turbine decelerates
from (B) to (C).
With 2 Lines
(Post-Disturbance)
With 1 Line
(Post-Disturbance)
Angle in degrees
30
60
90
120
28
150
180
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A three phase fault occurs near the generator:
The power supplied by the turbine remains constant
during the dynamic period.
The rotor speed increases storing the energy developed
by the turbine during the time that the generator output is
zero.
The δ between the turbine mechanical power and the
electrical power is called the accelerating power.
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Loss of Synchronization
TWO LINES IN
SERVICE
Generator at 600 MW
Power
90
180
270
360
450
Extended Power Angle Curve during Loss of Synchronism
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30
Loss of Synchronization
Seconds after an instability event occurs: The fault accelerates the generator sufficiently to
make it unstable. Where the decelerating area is less than the accelerating area.
Power
90
180
270
360
450
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
The generator goes through a deceleration area. Once the generator goes beyond the initial
deceleration area: The turbine mechanical power becomes greater than the electrical power out of the
generator.
Deceleration Area
Power
90
180
270
360
450
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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32
Loss of Synchronization
The generator begins accelerating again. As it does the electrical power out of the
generator goes to zero as the generator rotor angle approaches 180 degrees
Deceleration Area
Acceleration Area
Power
90
180
270
360
450
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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33
Loss of Synchronization
At 180 Degrees: The generator is accelerating very rapidly. Beyond this point the
generator electric power reverses and flows into the generator.
Deceleration Area
Acceleration Area
Power
90
180
270
360
450
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
The acceleration continues: Through the negative part of the power angle curve. From point (F)
through point (C’) the electrical power into the generator adds to the mechanical power from the
turbine and accelerates the rotor assembly to an extremely high rate.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
The acceleration continues: At this point the generator begins to act like an induction
generator running ahead of system speed.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Governor begins to reduce turbine
mechanical power
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
Loss of Synchronization: If the generator is not tripped quickly It will begin to run out of
step with the system.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Governor begins to reduce turbine
mechanical power
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
Loss of Synchronization: With reduced power from the turbine and with “induction power” flowing
out of the generator the generator will experience rapid reversals in “synchronizing” power.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Governor begins to reduce turbine
mechanical power
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
Loss of Synchronization: A generator that experiences these kind of rapid power reversals is
said to be “slipping poles”.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Governor begins to reduce turbine
mechanical power
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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Loss of Synchronization
Loss of Synchronization: At this point the magnetic bonds between rotor and stator are too
weak to keep the generator electrically connected to the system.
Deceleration Area
Power
(F)
90
(C’)
180
270
360
450
Governor begins to reduce turbine
mechanical power
Acceleration Area
Extended Power Angle Curve during Loss of Synchronism
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40
Recognizing a Voltage Collapse
• Voltage control problems are not new to the utility industry but the
problems in the past were primarily associated with the transfer of
power from remote generation sites to load centers.
• The main symptoms of voltage collapse are – low voltage profiles,
heavy reactive power flows, inadequate reactive support, and
heavily loaded systems.
• The collapse is often precipitated by low-probability single or
multiple contingencies.
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Recognizing a Voltage Collapse
- In high voltage transmission systems, the
inductive reactance of a line is typically much
greater than the resistance of the line
Gen Station A
-It is very difficult to transfer reactive power
long distances. When attempts are made, the
reactive losses are often so large that system
voltages fall as reactive power reserves are
used up.
Load Center
And Gen
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42
Recognizing a Voltage Collapse
Gen Station A
1.0 % PU
345KV
Generation MW Generation MVAR
9000
1150
Load MW
Load MVAR
6000
1000
3000MW
To transfer 3000MW the lines
require 300MVARs
1.0 % PU
345KV
Generation MW Generation MVAR
3000
Load MW
6000
Load Center
And Gen
1150
Load MVAR
1000
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43
Recognizing a Voltage Collapse
Summer
Gen Station A
1.0 % PU
345KV
Generation MW Generation MVAR
9000
9600
1150
1262
Load MW
Load MVAR
6000
1000
3600MW
To transfer 3600MW the lines now
require 362MVARs
1.0 % PU
345KV
Generation MW Generation MVAR
3000
Load MW
6000
6600
Load Center
And Gen
1150
1200
Load MVAR
1000
1100
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44
Recognizing a Voltage Collapse
Summer + in Texas
Gen Station A
1.0 % PU
345KV
Generation MW Generation MVAR
9000
9600
10,500
1262
1498
Load MW
Load MVAR
6000
1000
4500MW
To transfer 4500MW the lines now
require 453MVARs
.97 % PU 334.7KV
1.0
345KV
Generation MW Generation MVAR
3000
Load MW
6600
7500
Load Center
And Gen
1200
Load MVAR
1100
1245
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Recognizing a Voltage Collapse
Summer + in Texas + Murphy's Crew is on
Gen Station A
1.0 % PU
345KV
Generation MW Generation MVAR
10,500
1498
Load MW
Load MVAR
6000
1000
.97 % PU 334.7KV
Generation MW Generation MVAR
3000
Load MW
7500
Load Center
And Gen
1200
Load MVAR
1245
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Recognizing a Voltage Collapse
• Transient Instability:
A voltage phase angle instability that occurs due to a slow-clearing
transmission system fault.
Transient instability occurs when a fault on the transmission system
near the generating plant is not cleared rapidly enough to avoid a
prolonged unbalance between mechanical and electrical output of
the generator.
• Dynamic Instability
Dynamic instability is the condition that occurs when a fast-acting
generator AVR control amplifies, rather than damps, some small low
frequency oscillations that can occur in a power system.
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• Dynamic Instability can occur anywhere the load is remote from the
generation.
• Fast excitation systems are important to improve transient stability,
however, a fast-responding excitation system can also contribute a
significant amount of negative damping.
• Fast excitation systems can reduce the natural damping torque of
the system, causing un-damped megawatt oscillations after a
disturbance such as a system fault. This type of event can occur if
the generator is interconnected to a weak system and loads are far
from the generating plant. (October 2006, Gibbons Creek)
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October 3rd 2006 College Station
Loss of Load Event
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Recognizing a Voltage Collapse
1. CCVT on 138 KV
Gibbons Creek to Roans
Prairie fails
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2. Relay failure leads to explosion of CCVT,
tripping of 3 345/138 KV transformers and clearing
of both 345 KV busses. Major 345 KV path for
North to Houston cut
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3. Loss of Gibbons Creek 345/138 KV transformers and area generation
left only one 138 KV line to supply Bryan/College Station area. Additional loss
of generation (1200 MW total) triggers NERC DCS event and LaaRs shed.
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Oscillation Range
LOCAL
Voltage Oscillation
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Voltage Oscillation: Atkins 69kv 10/3/2006
17:10~10/3/2006 18:30
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Voltage Oscillation: HLK 69kv
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Voltage Oscillation: JEWET 69kv
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Possible Reason
• The voltage oscillation (instability) only existed around
the College Station area and the voltage oscillation
frequency was small(2~3mins/cycle), the transient
stability, dynamic stability issues can be excluded.
• The reason for this voltage oscillation issue is possibly
due to a “weak link” between College Station and the
rest of ERCOT system, which is a steady state voltage
stability issue.
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Possible Reason
• Motor load may try to reconnect to the system when the
voltage level reaches normal levels, however, the
starting of the motor/load would consume high reactive
power, which would then decrease voltage. The voltage
oscillation was most likely due to the interaction of motor
load.
• The recorded historical data showed this kind of
scenario.
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Atkins 69kv Load vs Voltage
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4. Line overload and difficulty maintaining voltage in area necessitated
Shedding of about 339 MW of load to prevent complete collapse of the area
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60
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Generation Response during Dynamic Periods
As the system begins to swing through these unstable dynamic
periods….
1.
The generator excitation system begins to respond in the opposite
direction.
2.
As the voltage on the system increases the voltage on the
excitation system decreases to hold the voltage set point on the
AVR. Conversely as the voltage on the system decreases the
voltage on the excitation system will increase to achieve the AVR
set point.
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3.
The voltage “swings” on the system will increase with every cycle
due to the over response of the AVR.
4.
During high system voltage:
The generator will swing to the leading side of the excitation “D”
curve consuming VARs from the system. If the under-excitation
relay setting is reached the unit will trip to prevent loss of
synchronization.
5.
During low system voltage:
The generator will swing to the lagging side of the excitation “D”
curve and produce VARs to increase system voltage. In this
situation if the over-excitation relay setting is reached the unit will
trip to prevent generator stator core damage.
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63
Recognizing a Voltage Collapse
Questions?
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Questions
1. List the symptoms an electric system displays
preceding a Voltage Collapse.
a) Low voltage profiles, heavy reactive power flows, inadequate
reactive support, and heavily loaded systems.
b) Transmission and generation outages
c) Generation de-ratings that create overloaded transmission
d) None of the above
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65
Questions
2. Identify the two types of system load and how each is
affected by low voltage.
a) Capacitive loads - During a low voltage period these loads assist
by providing voltage support.
Resistive loads - During low voltage conditions resistive current will
increase.
b) Resistive loads - During a voltage dip resistive load current will
decrease.
Inductive (Motor) loads - During a voltage dip motor load current
will increase - the lower the voltage, the more current they draw.
c) Load is not affected by low voltage
d) Inductive loads - During a voltage dip motor load current will
decrease – the lower the voltage, the less current they draw.
Capacitive loads - During a low voltage period these loads
decrease the voltage on the transmission system.
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Questions
3. Dynamic Instability is defined as:
a) Dynamic instability is the condition that occurs when a slow-acting
generator AVR amplifies a small low frequency oscillation.
b) Dynamic instability is the ability of the AVR to respond to a fastacting small low frequency disturbance.
c) Dynamic instability is the condition that occurs when a fast-acting
generator AVR control amplifies, rather than damps, some small
low frequency oscillations that can occur in a power system.
d) Dynamic instability is the condition that occurs when the stability of
the system becomes transient.
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4. IEEE defines Voltage Stability as:
a) The ability of a system to maintain voltage so that when load is
increased, power will increase, so that both power and voltage
are controllable.
b) The ability to control voltage with static devises on the
transmission system to prevent the use of dynamic control
from generators.
c) The ability of the system to maintain stable voltage within
voltage parameters.
d) The stability of generation after a system disturbance.
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5. IEEE defines Voltage Collapse as:
a) The process by which voltage instability leads to loss of
voltage in a significant part of the system.
b) The point at which voltage drops to a point where it is no
longer controllable by switchable devices.
c) The voltage increases to a point where it becomes
unstable and begins tripping generation.
d) Voltage collapse on the system that causes generation to
over speed and slip a pole.
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