The Effect of Delayed Zero Crossings Following a Short Circuit on

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The Effect of Delayed Zero Crossings Following a Short Circuit on System
Stability
James Gallagher*, Neil McDonagh and William Phang
ESB International
Ireland
SUMMARY
Fundamental to a circuit breaker is that it can only
break a fault current when the alternating current
crosses the zero. A fault current is made up of a DC
and AC component. If the DC component is
sufficiently large, there will be a large delay before
the current crosses the zero and therefore a large
delay before the circuit breaker can break the fault
current. This delay can be of the order of hundreds of
milliseconds.
The issue of delayed zero crossings following a short
circuit has been well documented in various literature
[1-3], however is still not adequately legislated for in
IEEE or IEC standards. This paper adds to the
existing body of knowledge by assessing the effects
of delayed zero crossings on system stability. This
paper uses a generic generator model to test for the
possibility of delayed current zeros following a short
circuit. This delay will then be used to assess the
effect on system stability using a generic bus system.
In cases where the DC component of the fault current
is larger than the AC component and the time
constant is quite large, the fault current wave form
may not cross zero for a long period of time. A
simplified LR circuit is shown in Figure 1.1.
Figure 1.1 – Switching of LR circuit
For a short circuit calculation in a simplified single
phase system the fault current may be defined as
shown in Equation 1 [4]. The waveform of a fault on
this system is shown in Figure 1.2. As can be seen,
the fault current is a combination of the AC
symmetrical component and the DC offset.
Keywords: Circuit Breaker – Zero Crossing– Stability
1. INTRODUCTION
Upon the occurrence of a fault, current of large
magnitude will flow. Initially, this current is a
combination of alternating and direct currents (AC
and DC). The DC component decays with time and
the time constant of this decay is determined by the
X/R ratio at the fault site. In the case of a high X/R
ratio, this decay time can be extremely large.
Asymmetrical Current
I
t
DC Component
AC Component
Figure 1.2 – Fault Current
 − E max


 ωL    − E max
 ωL  
i(t ) = e −( R / L )t 
sinϕ − tan −1 
sinωt + ϕ − tan −1 
  + 
 
2
2
2
2
2
2
R
 R + ω L

   R + ω L
 R  


*ESB International, Stephen Court, 18-21 St Stephen’s Green, Dublin 2, Ireland
Equation 1
If the simple system from Figure 1.1 is connected to a
load as illustrated in Figure 1.3, an initial pre-fault
current component is considered in the DC offset.
Load
Figure 1.3 – LR Circuit Feeding Load
This additional term in the DC offset will be of the
form:
e −( R / L )t I 0
Equation 2
Where I0 = pre-fault current flowing in the load
This I0 term is dependant on the current flowing in
the load and if it is in phase with the DC component,
from Equation 1 it will result in the DC component
being greater than the AC component which will
result in a delayed zero crossing. The longest delayed
zero crossings are associated with capacitive loads.
That is when generators are operating in the underexcited condition.
A waveform plot displaying a delayed zero crossing
is shown in Figure 1.4.
I
t
Delayed zero crossing
Figure 1.4 – Delayed Zero Crossing
1.1 Worst Case Delayed Zero Crossings
In reference to delayed zero crossings the worst case
scenario is a two phase fault that develops into a
three-phase fault, while generators are operating in an
under excited condition. This fault starts as a line to
line fault which initiates at the zero crossing of the
line to line voltage, which then develops into a threephase fault 5ms later. The worst case scenario occurs
when there is negligible resistance in the fault path,
so ideally this fault is a bolted fault. The occurrence
of this scenario is highly improbable but it is still
possible. One of the only instances in which this may
happen is if a section of network within the station is
outaged with earths applied, and the circuit breaker is
closed. Two poles of the circuit breaker must close in
unison at the zero point of the phase to phase voltage
between those two phases with the remaining pole
closing 5ms later. This sequence of events will result
in the largest DC offset and the longest zero crossing
delay times.
Significant delay times can also occur for other types
of faults such as simultaneous three-phase faults.
1.2 Circuit Breaker Operation
When a circuit breaker attempts to open, the current
flowing through the breaker must cross zero or close
to zero depending upon current chopping
characteristics. If there is no zero crossing, an arc will
form in the arc channel of the circuit breaker. This
arc will have a variable resistance, at medium voltage
levels the per unit resistance of this arc is sufficient to
change the X/R ratio in the fault path and bring about
a zero crossing. For high voltage circuit breakers the
arc resistance is much less effective in bringing about
a zero crossing, meaning that it may be difficult to
interrupt faults on the HV system near to generators.
If a circuit breaker attempts to break a current that
does not cross zero, it risks forming an arc that will
not be broken. This will destroy the circuit breaker,
endanger the lives of people in the vicinity of the
station, items of plant connected to the system, and
threaten the stability of the whole system. There is no
straightforward solution to this issue. The clearest
way to ensure that the ability of the circuit breaker to
break the fault current is not compromised is to
decrease the X/R ratio in the path feeding the fault,
delay the opening time of the breaker or, limit the
operation of generators.
1.3 Effects on Stability
It is possible that the fault on the system cannot be
cleared or it may be purposely not cleared until a zero
crossing has occurred. This zero crossing may not
occur for several hundred milliseconds. A fault on the
system for this long could have detrimental effects on
the stability of the system. The system can be studied
in terms of both voltage stability and transient angle
stability. Voltage instability is the result of a
progressive decline in voltage due to a disturbance.
Voltage stability issues are associated with the
transfer of active and reactive power over a highly
inductive network. It is also influenced by generator
and load characteristics, tap changer action and
reactive power compensation devices. Transient
stability analyses the ability of a power system to
maintain synchronous operation when subjected to a
transient disturbance. Transient stability is influenced
by fault clearance time, post-fault transmission
system reactance, generator loading, output,
reactance and inertia [5].
2. ANALYSIS
In order to assess the effects of delayed zero
crossings, a number of software models were
constructed. These models were used to both
calculate the delayed zero crossings and examine the
effect of those delayed zero crossings on stability. An
EMTP – ATP (Alternative Transients program)
model was used to calculate the delayed zero
crossings time delay. A PSSTME (Power System
Simulator for Engineers) model was used in order to
carry out dynamic simulations and analyse the effect
on system stability. For the purpose of this analysis, a
generic bus system was used to provide an example
of the effect of delayed zero crossings.
Sim ultaneous Fault, No Circuit Breaker, ST1, 168.5MW 120.3MVAr Under-Excited
4500
[A]
326ms
3400
2300
1200
100
-1000
0.0
0.1
0.2
0.3
0.4
[s]
0.5
Figure 3.1 – Delayed Zero Crossing Waveform
2.1 Voltage Stability Test System
This paper presents a relatively small system for the
analysis of voltage collapse issues. It is based on the
1979 IEEE Reliability Test System [6]. Modifications
were made to this system to make it more suitable for
voltage stability analysis[6].
Figure 3.2 shows zero crossing delay times for a
variety of machine operating points. The operating
points are grouped into regions according to the delay
time for zero crossing to occur.
This test system contains the following elements:
Region 1: time <= 100ms
Region 2: 100 < time <= 200ms
Region 3: time > 200ms
The total load of the system is 3,200MW.
Region 3
Delayed Zero Crossing
Region 2
400
350
300
250
MW
Network
Amount on
Element
Test System
230kV bus
14
138kV bus
10
18kV bus
34
13.8kV bus
17
230kV Feeder
21
138kV Feeder
13
Generators
32
SVC
2
Table 2.1 – Test System Elements
200
150
Limits
t<=100ms
100
100ms<t<=200ms
t>200ms
50
0
-150
-100
-50
0
MVAr
2.2 Delayed Zero Crossing Test System
A model of a single generator and step-up
transformer was built using software package EMTP
– ATP. Generic gas turbine generator and transformer
models were used. This test system was used to
calculate the duration of the fault the be applied on
the voltage stability test system.
2.3 Fault Scenario
As discussed in Section 1.1, the worst case fault
scenario is a two phase fault that develops into a
three-phase fault. Delays for simultaneous threephase fault are also of a similar order, although
generally shorter. In this paper only simultaneous
three-phase faults are analysed.
3. DELAYED ZERO CROSSING RESULTS
Zero crossing times for various machine operating
points were calculated in ATP for the generic
generator. It was found that a delay time of 326ms
from fault inception until fault zero crossing was
calculated for the machine operating point of
168.5MW, 120.3 MVAr under-excited. For this
calculation, neither circuit breaker operation nor arc
resistance were modelled. Figure 3.1 shows the
waveform for one of the faulted phases.
50
100
Region 1
150
Figure 3.2 – Operating Points and Delays
4. VOLTAGE STABILITY RESULTS
Simultaneous three-phase faults sustained for various
time lengths were applied at ASTOR 230kV bus (bus
number 118). It is assumed that the fault is sustained
at the 230kV bus for an extended period of time as
the circuit breakers cannot clear the fault until a zero
crossing occurs.
4.1 Effect of 100ms Delay
A 100ms delay corresponds to a machine operating
point of 51.3 MW, 23.9 MVAr under-excited, see
Region 1 in Figure 3.2.
Figure 4.1 shows the bus voltage for fault at bus 118.
A three-phase fault is applied at 1s and sustained for
0.1s before being cleared. When the fault is cleared,
the system voltages are restored to the pre-fault
values and the oscillations die out.
Figure 4.1 – Faulted Bus Voltage
Figure 4.4 – Active Power Output
Figures 4.2 and 4.3 show the output of the SVCs at
bus 10114 (-50/+200 MVAr) and 10116 (-50/+100
MVAr).
Both increase to their maximum reactive power
output value on occurrence of the fault and stay at
this value for approx 0.5s after the fault is cleared.
They both then decrease to a steady state value.
The reactive power output of the machine initially
increases. However, the reactive power then
decreases back to its pre-fault value after the fault has
been cleared.
Figure 4.5 – Reactive Power Output
Figure 4.2 – SVC at Bus 10114
Figure 4.3 - SVC at Bus 10116
Figures 4.4 and 4.5 show the active and reactive
power outputs of the machine feeding the faulted bus
118. On occurrence of the fault, the active power
output of the machine drops but it increases back to
its pre-fault steady state value after approximately
10-15 seconds.
As can be seen from Figures 4.1 - 4.5, the system can
cope with a three-phase fault of 100ms duration. That
is, a fault in which a zero crossing occurs and
therefore circuit breakers can act within 100ms.
Section 4.2 presents a situation where the fault is not
cleared within the first 100ms.
4.2 Effect of 326ms Delay
Figure 4.6 shows the voltage at bus 118 where a fault
with a delayed zero crossing is applied. A time of
326ms for the delayed zero crossing is used. This is
the maximum time calculated using the Delayed Zero
Crossing Test System for a simultaneous three-phase
fault. The fault is applied at 1s and sustained until
1.326s when it is cleared.
As can be seen from Figure 4.6, the system cannot
recover the voltages when a fault is sustained for this
long and the system loses stability.
Figure 4.6 - Faulted Bus Voltage
Figure 4.9 - Active Power Output
Figures 4.7 and 4.8 show the output of the SVCs at
bus 10106 (-50/+100 MVAr) and 10114 (-50/+200
MVAr). On occurrence of the fault, both SVCs
increase to their maximum output values of reactive
power to raise the system voltages. However, this is
not enough to prevent the system becoming unstable.
Figure 4.10 - Reactive Power Output
Figure 4.7 - SVC at Bus 10114
Figures 4.6-4.10 demonstrate that a small power
system cannot cope with a fault being sustained for
326ms. It is important, therefore, to prevent the
occurrence of a fault with delayed zero crossings.
Section 5 presents some possible mitigating actions
to prevent such a fault occurring.
5. DISCUSSION
In essence the phenomenon of delayed zero crossings
is not a new problem but is becoming more of an
issue due to the advent of low(er) loss generators and
transformers.
Unfortunately the matter of delayed zero crossings as
a result of a fault on a high voltage system is an issue
that is not adequately dealt with by international
standards [7]. Therefore, there is no standard with
which HV manufacturers can design and test their
equipment adequately.
Some mitigating actions are discussed in Sections
5.1-5.3.
Figure 4.8 - SVC at Bus 10116
Figures 4.9 and 4.10 show the active and reactive
power outputs of the machine connected to bus 118.
As can be seen the system becomes unstable
approximately 0.5s after the fault is cleared.
5.1 Limit Operating Range of Generator
If this were considered a viable solution, results
suggest that the generator would have to be limited
from operating with a leading power factor.
This may have serious implications for the control of
voltages in the region and it is questionable that a
system operator would allow such a solution.
If there is a requirement for the plant to provide
voltage control to the system, an adequately sized
reactive compensation provided by a shunt reactor,
SVC or STATCOM, would remove the need for the
generators at the plant to operate in the under-excited
mode.
5.2 Insert Series Resistor in Generator Circuit
One of the most straightforward ways to eliminate
delayed zero crossing is to include additional
resistance in the circuit. This additional resistance
may be provided in a number of ways but the most
straightforward would be to increase the resistive
losses in the transformer. This action will decrease
the X/R ratio and improve the ability of the circuit
breakers in the clearing of worst case possible faults.
It also must be worth noting that this action will
reduce the overall efficiency of the plant. There may
also be issues surrounding the adequate dissipation of
heat from a transformer with increased resistive
losses.
5.3 Protection Coordination
It may also be of value to consider the backup
protection provided by the MV system. With regards
to the adequacy of medium voltage breakers to break
faults resulting in delayed zero crossings [8] states
the following:
“It is generally accepted that the generator circuit
breaker will be required, during its life, to interrupt
short-circuit currents from the generator-source with
delayed current zeros. ……. The determining arc
voltage model is derived from tests with comparable
magnitudes of current.”
As this is the case, it may be prudent to rely more on
MV circuit breakers to break faults on the HV
system. This may be achieved by the adequate
coordination of protection provided by the MV
generator circuit breakers.
Reducing the backup protection time will reduce the
possibility of damage to the high voltage circuit
breaker and other plant. However, the employment of
this solution may result in an increased number of
generator trips.
5.4 Operational Considerations
The phenomenon of delayed zero crossings occurs as
a result of a three-phase fault. Two of the most
probable ways in which a three-phase fault can occur
is a result of lightning or leaving earths connected to
a line or busbar. An exhaustive assessment of a
particular site may conclude that the actual
probability of occurrence of such a three-phase fault
may be at an acceptable level without the need for
additional mitigating actions.
6. CONCLUSIONS
From the simulations on the voltage stability test
system, it was found that the system remained stable
for a fault on the system for 100ms. However, if a
fault is sustained on the system for 326ms, the system
loses stability. Calculations from the delayed zero test
system showed that fault of a duration of 326ms
could occur as a result of a delayed zero crossing. It
is clear that this would have drastic consequences for
a small/medium sized power system.
If it is required that plant be designed to be protected
from this phenomenon, the following mitigating
actions may prevent a loss of system stability in the
event of a three-phase fault:
•
•
•
Limit operating range of generator in
conjunction with the use of a SVC or
STATCOM
Use of series resistance
Protection coordination
Naturally, there are drawbacks associated with each
of these measures and each one would have to be
studied individually to assess its effectiveness. As
part of this detailed assessment, the operational
aspects of a particular plant would also have to be
considered.
REFERENCES
[1]. H. Hamada et al “Sever Duties on HighVoltage Circuit Breakers Observed in
Recent Power Systems” Cigre 13-103
Session 2002
[2]. S. Henschel et al “Breaking Capability of High
Voltage SF6 Breakers in Power Plants” XI
Symposium of Specialists in electric
Operational and Expansions Planning,
Brazil 2009.
[3]. B. Kulicke et al “Clearance of Short-Circuits
with Delayed Current Zeros in the ITAIPU
550kV Substation”
[4]. L Van Der Sluis, Transients in Power Systems,
John Wiley and Sons, 2001
[5]. P. Kundur, Power System Stability and
Control, McGraw-Hill, 1994
[6]. L. T. G. Lima et al “PSS®E Test System for
Voltage Collapse Analysis”
[7]. R.E Cossé et al “IEC Medium Voltage Circuit
Breaker Interruption Ratings – Unstated
Short-Circuit Considerations
[8]. IEEE Std C37.013™-1997 (R2008) IEEE
Standard for AC High-Voltage Generator
Circuit Breakers Rated on a Symmetrical
Current Basis
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