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unit_2.ppt
1. UNIT II SWITCHING TRANSIENTS
Over voltages due to switching transients -
resistance switching and the equivalent circuit for
interrupting the resistor current - load switching and
equivalent circuit - waveforms for transient voltage
across the load and the switch - normal and abnormal
switching transients. Current suppression - current
chopping – effect ive equivalent circuit. Capacitance
switching - effect of source regulation - capacitance
switching with a restrike, with multiple restrikes.
Illustration for multiple restriking transients - ferro
resonance.
2.
3.
4. • A transient occurs in the power system when the network
changes from one steady state into another.
• The majority of power system transients is, however, the
result of a switching action. Load break switches and
disconnectors switch off and switch on parts of the
network under load and no-load conditions.
• Fuses and circuit breakers interrupt higher currents and
clear short-circuit currents flowing in faulted parts of the
system.
• The time period when transient voltage and current
oscillations occur is in the range of microseconds to
milliseconds.
INTRODUCTION
5. OVERVOLTAGE
• An overvoltage is an increase in the r.m.s. ac
voltage greater than 110 % at the power
frequency for a duration longer than 1 min.
• It is a result of load switching.
Eg : Switching OFF a Large load
Energizing a capacitor bank
6. • Switchings in a power system occur
frequently.
• A variety of switchings are performed for
routine operations or automatically by
control and protection systems.
• Typical switchings are as follows:
– Lines (transmission or distribution)
– Cables
– Shunt/series capacitors
– Shunt reactors
– Transformers
– Generators/motors
7. OVER VOLTAGES
• The causes of power system over voltages
are numerous and the waveforms are
complex.
• It is customary to classify the transients on
the basis of frequency content of the
waveforms.
• In this sense, the following three broad
categories are defined:
–Power frequency overvoltages
–Switching overvoltages
–Lightning overvoltages
10. How does Resistance Switching
Work
• Let us consider a circuit and connect a shunt
resistance R across the contacts of the breaker
as shown . Suppose a fault occurs on the line.
Because of the occurrence of fault, the
contacts of breaker will open and an arc will
stuck between the contacts.
11. • Since the contacts of breaker are shunted by resistance R, therefore a
part of arc current will flow through this resistance R. Due to this the
magnitude of arc current will reduce which in turn will result in increase
in the rate of de-ionization of arc path. In this way, the arc resistance
increases. This increased arc resistance leads to further increases in the
current through the shunt resistance R.
• Thus again, the arc current will reduce and hence the arc resistance
increases. This cumulative process will continue till the magnitude of
arc current becomes so low that it is not able to maintain the arc. Thus
the arc extinguishes and the current is interrupted by the breaker.
• Resistance Switching also reduces the oscillatory growth of Re-striking
Voltage. The natural frequency of the circuit shown above is given by
fn = (1/2π)√(1/LC – 1/4R2C2)
• The main role of shunt resistance R is to limit the growth of re-strike
voltage and cause it to grow exponentially up to recovery voltage. If the
value of R is so selected that the circuit becomes critically damped then
re-strike voltage rises exponentially till recovery voltage is reached.
14. OBJECTIVES
i) To reduce switching surges and over
voltages
ii) For potential control access multi-breakers
per phase in the high voltage breakers
iii) To reduce natural frequencies effects and
breaker recovery voltage
15. NEED FOR RESISTANCE SWITCHING
• The shunt resistors connected across circuit
breaker have two functions:
i) To distribute the transient recovery voltage
more uniformly across the several breaks.
ii) To reduce the severity of transient recovery
voltage at the time of interruption by
introducing damping into oscillation.
16. • Thus the advantages of resistance switching
are as follows:
• It reduces the rate of rise of re-striking voltage
and the peak value of re-striking voltage.
• Resistance Switching helps to reduce the
voltage transient surge during current
chopping and capacitive current breaking.
30. Normal switching transients are circumstances in
which voltage or current within the normal peak
values Closing switch or circuit in a dominantly
capacitive or inductive network results in inrush
currents which can cause problems for the
protection system.
Abnormal switching transients are circumstances
in which voltage or current are far in excess of twice
in normal peak values. Insulation of high voltage
circuit breakers typically can over voltages up to 2.5
times over its normal voltage.
31.
32.
33. Capacitor Switching
• Capacitor switching is one of the most common
switching events on utility systems.
• Capacitors are used to provide reactive power (in
units of vars) to correct the power factor, which
reduces losses and supports the voltage on the
system.
• They are a very economical and generally
trouble-free means of accomplishing these goals.
• Alternative methods such as the use of rotating
machines and electronic var compensators are
much more costly or have high maintenance
costs.
34. One drawback to the use of capacitors is that
they yield oscillatory transients when switched.
Some capacitors are energized all the time (a
fixed bank), while others are switched according
to load levels.
Various control means, including time,
temperature, voltage, current, and reactive
power, are used to determine when the
capacitors are switched.
It is common for controls to combine two or
more of these functions, such as temperature
with voltage override.
35. • One of the common symptoms of power quality problems
related to utility capacitor-switching overvoltages is that the
problems appear at nearly the same time each day.
• On distribution feeders with industrial loads, capacitors are
frequently switched by time clock in anticipation of an increase
in load with the beginning of the working day.
• Common problems are adjustable speed-drive trips and
malfunctions of other electronically controlled load equipment
that occur without a noticeable blinking of the lights or impact
on other, more conventional loads.
36. Restrikes During Capacitor
De-Energizing
When a grounded capacitor bank is energized using a
mechanical oil filled switch, the switching is most likely
to occur at or near the system voltage peak.
At this instant, the capacitor voltage is zero while the
system voltage is near maximum.
The largest potential difference between contacts is 1.0
pu.
Due to the slow-moving nature of the mechanical
switch and the large potential difference, insulation
across the switch contacts tends to break down giving
rise to an electric arc.
It allows capacitive current to flow, thereby energizing
the capacitor before the actual switch contacts close or
touch. This phenomenon is known as pre-strike.
37. It occurs in many types of switches without
closing control.
It is usually considered as a natural
behavior of the switch and should not raise
any concern because capacitor voltage prior
to energizing is negligible.
The resulting peak transient voltage is no
worse than that when the capacitor is
intentionally energized at the system
voltage peak.
38. On the other hand, restrikes (or reignitions) during
capacitor deenergizing do rise concerns.
Restrikes or reignitions occur when arcs between parting
contacts are re-established after initial current extinction
causing unintended re-energizing of the capacitor.
Because of the trapped charges following the initial
extinction, significant overvoltage well above 2 pu can
result.
It should be noted that a successful capacitor deenergizing
(i.e., no restrike) does not normally yield any transient
overvoltage.
39. Restrikes can occur multiple times and in
succession over a short time period.
It can also be intermittent over a longer
time span. In either circumstance, a restrike
poses problems to the power system as well
as to the switching device itself.
40. Capacitance Switching Off
• Disconnect: C /unload Transmission
lines
• Concerns: reignite/restrike in
opening
• Chance low, Cap. Sw. frequent
• Cap fully charged
• Half Cycle VCB=2 Vp
42. Discussion Cap. Sw. Off
• In fact Vc>Vsys Ferranti Rise
• Vsource_side decrease to Vsys
• There is a ∆V change (however,exist in weak systems)
• Discon. a C.B. in lower side of step down Transformer
supplying an unloaded cable
• Current in Cap. Sw. is freq. small and it is
possible to disconnect it In first zero
-- with small contact sep., 2 V appear across contacts
--- increased possibility of restrike (small separation)
• Oscillating to new voltage with f0=1/2Π√LC
• I(restrike)=2Vp/√L/C sinω0t
• Transient peak of 3 Vp
46. FERRO RESONANCE - DEFINITION
• The term Ferro resonance refers to a special kind of
resonance that involves capacitance and iron-core
inductance. The most common condition in which it
causes disturbances is when the magnetizing
impedance of a transformer is placed in series with a
system capacitor. This happens when there is an
open-phase conductor. Under controlled conditions,
ferro resonance can be exploited for useful purpose
such as in a constant-voltage transformer
48. Ferro resonance- Occurence
• Nonlinear resonance between unloaded transformer
magnetizing branch and a series capacitor
• May occur when distribution transformers fed from
cables are switched one phase at a time
• Especially a problem on ungrounded star-delta
transformers
• May occur any time an unloaded iron-core coil is in
series with a capacitor
49. 2. Terms and definitions 49
Long-duration overvoltage caused by
ferroresonance of transformer
50. Ferro resonance is different than resonance
in linear system elements. In linear systems,
resonance results in high sinusoidal
voltages and currents of the resonant
frequency.
Ferro resonance can also result in high
voltages and currents, but the resulting
waveforms are usually irregular and chaotic
in shape.
51. The most common events leading ferro resonance are,
Manual switching of an unloaded, cable-fed,
three-phase transformer where only one phase
is closed.
Ferroresonance may be noted when the first
phase is closed upon energization or before the
last phase is opened on de-energization.
53. Possible effects of ferroresonance
• Overvoltage
• Audible noise
• Overheating
• Sustained overvoltage => arrester failure
• Flicker due to erratic voltage
• Subharmonic voltages (e.g., at 1/3 or 1/2 of
normal frequency)
• Chaotic response
54. Summary
• In Unit 2, we have seen
switching transient- Resistance switching
Load switching
Capacitor switching
Current chopping
Normal and Abnormal switching transient
Current suppression
Ferro resonance
