Time Current Characteristic Curves for Selective Coordination

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.

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 y-axis.
• 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?

Selective Coordination is defined as a method of adjusting the opening times of overcurrent protection devices so that the fuses or breakers nearest to the faults open first. 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 over-current 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.B-5). 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?

Selective coordination is accomplished by adjusting and rearranging the time current curves of protective devices such that their settings or curves have minimum or no overlapping. Each protective device is dedicated to a particular zone. They 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.

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.

1. 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.

   

2. 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.

   

   

3. 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 before it as well. So, selectivity is maintained in this scenario.

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 time-current 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.

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:

Definition: The transformer damage curve describes the point at which a transformer can suffer thermal (heating) or mechanical (vibration) damage as a result of an overcurrent event.

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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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.