Last updated: Oct 22, 2020
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Time Current Characteristic Curves for
Selective Coordination
Time Current Characteristic Curves for
Selective Coordination
Published Date: Oct 22, 2020 Abdur Rehman
Time Current Characteristic Curves play a significant role in achieving
proper protection coordination among the electrical safety devices.
Learn more as we cover basics of power system protection, TCCs for the
solid state and thermal magnetic trip, importance, procedure and rules
of selective coordination here.
Objective of Power System Protection:
The primary objective of power system protection is to sense the fault
or any abnormal condition which may cause the system to malfunction
or causes complete outage of power and isolate it from the healthy
section.
Studies are required for protecting the crucial power system
equipment. Selective and protection coordination is done with the help
of Time Current Curves (TCCs). This article discusses the significance
of power protection coordination and how time current curves are used
for the purpose of selective coordination.
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Principles of Power System Protection:
When designing a power system protection scheme, an engineer must
pay attention to the following characteristics so that our protection
system provides optimum functionality.
Sensitivity: Protection equipment should be sensitive in accurately
detecting all kinds of faults.
Speed: Speed in tripping (cutting of supply from healthy region)
Economics: Less expensive. Should not cost greater than 25% of
the overall cost.
Simplicity: should not make the overall system to look complex
Selectivity: identifying the correct faulty part so that the least part
gets affected. For example, a university has its main breaker and
each department has its own breakers, suppose if a fault occurs in
a department, It should not trip the main breaker of the university,
instead the main breaker of that department should trips.
Time Current Curves (TCCs)
Fault intensity in power systems is proportional to the magnitude of
current. It is desired that as fault current increases the Fault Clearing
Time or FCT should be decreased. To ensure that all the downstream
and upstream protective devices are coordinated, current versus time (I
versus t) curve is used which is also known as TCC or Time Current
Curve.
Following are the characteristics of TCCs:
In a TCC, current is mentioned on the x-axis while time on the yaxis.
A TCC is plotted on a logarithmic scale so that all values of current
and time are easily incorporated. For example: in a system, a
minimum fault of 100 A should be cleared within 10 s and for a
system with a maximum fault of 5000 A it must be cleared within
50 ms. Logarithmic scale in TCC ensures that both extreme values
of current and time are present.
Relay curves are sharper and thinner than fuse and breakers
because relays are only used to sense a fault and then issue a trip
signal to the breakers. They are typically used in MV and HV
systems. Check out Power System Protection
Fundamentals Course in which we briefly discussed "Types of
protective relays & design requirements".
TCC of a Circuit Breaker:
Solid State Trip:
Below are some key points that are reflected in graph shown above.
Long Time Ampere Rating: It is the continuous current rating at
which breaker shows no tripping. For example, a breaker is rated
at 1000 A and maximum current that will flow through the breaker
is 800 A. Therefore, long-time ampere setting will be adjusted to
800 A.
Long Time Delay: This setting refers to delay due to inrush current
of transformer and starting current of motor. This delay is given in
the form of slope.
Short Time Pickup: It is 1.5 to 10 times the long time ampere
rating. The setting at which breaker tends to trip after some delay.
Short Time Delay: Delay given due to check if downstream devices
cleared the fault so no trip issues or else after reaching delay
breaker trips. It has two settings available
Instantaneous Pickup: Used when tripping is required without any
delay. Its setting can vary from 2 to 40 times of long-time ampere
rating.
Thermal Magnetic Trip:
As seen in the graph below, the breaker curve has a wide thickness. This
thickness in the graph has its own meaning which is described by two
terms known as:
Minimum Clearing Time: It is the time at which breaker senses a
fault.
Maximum Clearing Time: It is the time at which breaker issues a
trip signal.
Thermal magnetic breakers have slightly different characteristic
graphs than electronic (solid state) breakers as they have only two
settings:
Delay Trip: This trip is due to overcurrent sense by thermal part of
the breaker. Bimetallic strip in the breaker heats up on high
current causing the contacts to break up after a delay. As the
current increases, heating goes on and overcurrent clearing time
decreases.
Instantaneous Trip: There is no intentional delay in tripping. The
magnetic part of the breaker senses high overcurrent or short
circuit and issues a trip signal.
What is Selective Coordination?
Complete selectivity means that the protective devices will minimize
the effect of a short circuit or other undesirable event on the power
system. The fuse or a circuit breaker closest to the fault opens without
opening the fuse or the circuit breaker that feeds it (from the upstream
side). So, you won't have the power outage if there is a fault somewhere
downstream.
According to NEC article 100, Selective coordination is defined as:
“Localization of an over-current condition to restrict outages to the
circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.”
In order to understand how protective devices are coordinated, let us
take an example:
Figure 1: Fault below CB5
The above figure shows a fault that occurs below circuit breaker 5 (C.B5). In this case, C.B-5 should be able to clear the fault in the least
possible time and no other breaker (in this case C.B-2 and C.B-1)
should trip during this time. In case, the breaker C.B-5, due to any
reason, does not clear the fault, then C.B-2 clears it after some delay
and if, due to any reason, C.B-2 is not able to clear fault, then C.B-1
issues a trip (which could be the worst case scenario).
How Selective Coordination is done?
Protective devices should only operate on faults that lie within their
“zone of protection”. When a fault occurs in a particular zone, the
device dedicated for its protection will sense the current and isolate the
fault from the remaining system.
However, if a fault occurs outside the zone of protection of a device,
then that device will only sense it but will not trip. Hence, by adjusting
and rearranging the time current curves of protective devices such that
their settings or curves have minimum or no overlapping, selective
coordination can be achieved.
Achieving Selective Coordination using ETAP:
For example, above shown is a simple part of a system for which we will
obtain TCC curves first and then will adjust curves so that we could
achieve coordination among all protection devices.
Select the part of system of which TCC is to be obtained. Then from
below shown module bar (Fig 01) we will select Star Protective
Protection Then as shown in Fig 2 we will select Create Star View.
After clicking, the below mentioned graph will appear in a pop-up
screen. Below mentioned graph is of CB 2 which is shaded red. In
this case this is the lowest most breaker so according to rules, it
must be at the leftmost position because we want it to trip first.
As CB-1(shaded red below) is the second last protective device
then its graph should be to the right of CB-2 Breaker, because we
want it to trip in case CB-1 fails to trip or if the fault occurs in its
zone. This situation is shown in figure below.
Rules of Selectivity:
Case 1:
The Use of Pickup Settings Figure 2 shows how curves with different
pickup values can be selective and illustrates the first rule of selectivity,
which is, two devices are selective if the downstream device curve is to
located to the left of the upstream device curve. This can only happen
when the pickup setting of the downstream device is set to a current
that is less than the pickup setting of the upstream device. Note that the
convention for time current curves is to end the rightmost portion of
the curve at the maximum fault current that the device will sense in the
power system its applied in. Increasing the pickup setting shifts the
curve toward the right of the graph. In the example, for any current up
to the maximum fault current of the left-hand curve, the curve on the
left will trip out before the curve on the right. Currents that exceed the
maximum current of the left-hand curve are not physically possible
and are sensed only by the device represented by the right-hand curve.
Fig. 2 – Creating selectivity by proper selection of pickup settings.
Case 2:
The Use of Delay Settings (Figure 3) shows how varying time delays can
provide selectivity. Increasing the time delay shifts the curve upwards
on the graph. Note that for all currents within the range of the curves,
the curve on the bottom will trip out before the curve above it. So, the
second rule of selectivity is that the downstream device must be placed
lower on the graph than the upstream device for the two devices to
operate selectively.
Fig. 3 – Creating selectivity by proper selection of delay settings.
Case 3:
Determining the selectivity of a set of time current curves is quite easy.
The curves should line up in from left to right or bottom to top in the
sequence of load to source. There should be no overlapping of the
curves nor should they cross each other. There should be sufficient
space separation between the curves (more on this later). The curves
can also indicate whether upstream devices provide backup protection.
This occurs when the left-most portion of the backup device extends
over into the range of currents of the preferred device.
In Figure 4, the devices line up as recommended. Note that as you
follow the three fault current levels through time, the device closest to
the load will finish it’s time delay first and trip before the other
breakers. If the device closest to the load fails to operate, the next
device upstream will trip after the additional time delay indicated and
before the other remaining device.
Fig. 4 – Identifying complete selectivity
Figure 5 offers an example of a system that is not selective at certain
current levels. Three fault locations and corresponding current levels
are shown using the colored symbols and arrows. Each breaker shown
is in a switchboard or panel that can contain other feeders or branches.
So, the tripping of either Breaker 1 or Breaker 2 will isolate much more
than the single load shown in the single-line diagram.
Let’s begin with the fault located at the green cross with fault current
signified by the green arrow. The fault location causes current flow
through all three breakers. But the current magnitude is high enough to
cause only breakers 1 and 3 to pick up. Breaker 3 will trip first and
isolate the fault, so the system appears to be selective. However, notice
that in a backup situation, Breaker 1 will trip rather than Breaker 2, and
result in an outage to more of the power system than necessary.
Fig. 5 – An example of a non-selective system
The fault shown by the blue cross is located on the incoming side of
breaker 3, so this breaker will have no current flowing through it.
Breakers 1 and 2 will sense this fault. Because of the crossing of the
curves of Breakers 1 and 2, Breaker 1 will trip first for this fault, which is
undesirable since it would be isolating more of the system than
necessary.
The fault shown by the yellow cross has a very high current which is
sensed by both breakers 1 and 2. In this case, the current level is high
enough to pass through the curves where Breakers 1 and 2 are selective
i.e. to the right of the intersection of their curves. Therefore, we can see
that breaker 2 will detect the current before breaker 1 and will trip
TCC of a Fuse:
Figure 4: TCC of a Fuse
Each fuse is represented by a band: the minimum melt characteristic
(solid line) and the total clear characteristics (hash line). The band
between the two lines represents the tolerance of that fuse under
specific test conditions. For a given overcurrent, a specific fuse, under
the same circumstances, will open at a time within the fuse’s timecurrent band. Also, fuses have an inverse time-current characteristic,
which means the greater the overcurrent, the faster they interrupt.
Cable Damage Curves:
Cable damage curve shows that how much current a cable can carry
without insulation damage and for how long it can withstand different
values of currents.
Figure 5: A typical cable damage curve
Full Load Amps: It is the continuous current or the rated current that
will be flowing through the cable, it is a load dependent quantity and a
cable must be sized such that it can easily carry this current.
Cable Ampacity: Also known as the Current Carrying Capacity, it is the
maximum current in Amperes which a cable can continuously carry
without damaging its insulation, or without exceeding its rated
temperature.
Figure 6: Protecting a cable
We ideally want that our circuit breaker trips and isolate the upstream
cables before they get damaged from any fault current. Therefore,
when drawing TCCs, we adjust our breaker curves to the left of the
cable damage curves. This indicates that the breaker will trip before the
fault current damages any of the cables.
A cable that is not selected in accordance with the system fault current
levels may get damaged easily, or an improperly sized cable may
overheat as well. So, selecting the right cable size and type is highly
critical in terms of maintenance cost, safety and reliability.
To learn more about how to select the proper cable, follow our blog
written on How to choose the most economic size and type of cable?
TCC of a Transformer:
The high starting current which a transformer draws to energize itself
is called the inrush current of a transformer. Tripping due to inrush
current is indeed a nuisance because we want are transformer to
continue operating after this and not trip.
We can plot this characteristic on a TCC as well. A circuit breaker should
ideally be to the right side and above of a transformer inrush curve.
This indicates that a circuit breaker will not trip under inrush current
conditions. If the breaker curve is to the left of an inrush curve then it
would indicate nuisance tripping.
Figure 7: Coordinating with transformer inrush and damage curves
Nuisance Tripping due to Inrush Current:
Sometimes, temporary high currents or overload conditions such as
transformer inrush current, motor inrush current, currents from motor
drives or even occasional surges occur in our system. They persist for a
short time, lasting about 10ms on average for transformer inrush while
a few seconds for motors.
However, it is not acceptable that our system treats these as faults.
Tripping under these conditions is known as nuisance tripping because
these conditions frequently occur in power systems and we do not want
our system to trip each time this happens.
Transformer Damage Curve:
"IEEE Guide C57.109-1993 (R2008) considers both thermal and
mechanical effects for external transformer through faults."
The transformer’s capability to withstand these effects is shown in
figure below.
Figure 8: Thermal capability curve of a transformer
I2t (I = amps, t = time) with unit Amp Squared seconds(A2S) is
proportional to the increase of thermal energy in a conductor resulting
from a constant current over time. In transformers an I2t value is
defined to show the thermal limits of their windings before damage
occurs.
Damage curves are also known as withstand curves. A breaker should
be coordinated with a damage curve on a TCC such that it protects the
device from currents that will damage it. Therefore, a breaker curve is
supposed to be to the left of a withstand curve and not overlap with it
so that our transformer is completely protected from all values of
currents that exceed its damage ratings.
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