ETAP Star Coordination on Focus | Detailed Example

The main objective of any protection system is to interrupt short-circuits as fast as possible. The means to achieve this range from the simplest and cheapest systems to the most complex and expensive schemes. Among these, overcurrent protection is often the simplest and the most inexpensive to employ.

A compliment to overcurrent protection is the procedure that looks at the importance of maintaining a very high level of continuity of service. The concept of overcurrent coordination ensures that only a minimum portion of the power system is interrupted from a fault or overload condition.

In our previous discussions about overcurrent coordination, we learned about the objectives of power system protection and the principles of coordination. These objectives are

• prevent human injury,
• limit equipment damage, and
• limit the extent of service interruption,

while the principles include

• selectivity,
• speed of operation,
• simplicity,
• sensitivity, and
• economics.

Next, we discussed the concept of zones of protection, primary and backup protection, local and remote backup, and basic considerations in a coordination study.

We went on to discuss overcurrent protection devices, their advantages and disadvantages, and their time-current curves. Our understanding of time-current curves was supplemented through a discussion of Coordination Time Intervals (CTIs).

Lastly, overcurrent coordination through the discrimination by time and current was highlighted in our discussion on the development of the Inverse Definite Minimum Time (IDMT) relay.

In this article, we will complete our overcurrent coordination series with a sample coordination study using ETAP Star Coordination. The example is taken from the book “Protective Relaying Principles and Application” by J. Blackburn.

The example shows a typical distribution substation with protective devices that include the power transformer high-side fuse, feeder circuit breaker, line recloser, and lateral fuses.

ETAP Star Coordination

The ETAP star coordination study module can be accessed from the mode toolbar by clicking the ‘Star – Protection & Coordination’ icon.

Before proceeding to the next section, it is recommended that you familiarize yourself with the software user interface. You can refer to this link to review the basic elements and toolbars available in the software, and the step-by-step process of modeling.

Building the One-line Diagram

After creating a new project, start by building the system one-line diagram. This is a straightforward procedure that involves selecting and connecting the power system devices in a single line. Nothing can be more simple.

Introducing the ‘Auto-Build’ Feature

For large systems, even a process as simple as building the one-line diagram could take quite some time. ETAP addressed this by introducing the ‘Auto-Build’ feature. ‘Auto-Build’ is a rule-based automated creation of a one-line diagram that includes automatic spacing and alignment.

To illustrate this, let us continue by building the one-line diagram shown in figure 1.

The default rules are already defined and working out-of-the-box when enabled so you won’t need to mess with them. However, if you have your own preferences, you may edit the rules to suit your needs.

After building the one-line diagram and inputting device/equipment parameters, your model should look like the figure below.

If you are interested in trying this sample project in ETAP star coordination, navigate to the device/equipment specifications section on the next page.

ETAP Star – Protection & Coordination

Begin the ETAP star coordination study by clicking the ‘Star – Protection & Coordination’ icon in the mode toolbar. To verify that you are in the coordination module, your ‘Study Mode Toolbar’ located on the right side of your screen should look like the figure shown in figure 5.

Fuse Ratings

The fuse ratings for this sample project were selected based on the lateral loads they serve. In addition, type ‘T’ (slow-acting) fuse-links were selected for lateral taps since they coordinate well with distribution transformer fuses which generally use the fast-acting type ‘K’. These ratings are shown in figure 1.

For the power transformer high-side fuse, the fuse rating is based on the transformer maximum rating (2nd forced-air cooled from class OA/FA/FA) of 25 MVA. An E-rated fuse is selected to provide current limiting protection.

To create a time-current characteristic plot in ETAP star coordination, select the devices or equipment from the one-line diagram and click the ‘Create Star View’ icon from the ‘Study Mode Toolbar’.

Figure 7 shows the time-current characteristic plot of the 125 A E-rated power fuse and the transformer through-fault damage curve.

In order to show and/or modify the information displayed on the TCC plot, open the transformer parameters window by double-clicking the transformer symbol on the one-line diagram, and navigate to the ‘Protection’ tab.

From the TCC plot in figure 7, the following can be inferred.

• The transformer inrush transient is located to the left of the fuse characteristic curve. This will provide security during transformer energization.
• The transformer through-fault damage curve is shifted to the left considering that the 115 kV side protection sees only 57.7% of the current resulting from a line-to-ground fault in the 13kV side. This is applicable to Delta-Wye connected transformers.
• The transformer is sufficiently protected from thermal damage. The fuse characteristic curve is located to the left of the shifted transformer damage curve.

Recloser Settings

The recloser in the example has a maximum load of 230 A. The phase overcurrent minimum operating current or pick-up value is set to more than twice the maximum load, at 560 A, to provide security for cold-load transients. On the other hand, the ground overcurrent minimum operating current or pick-up value is set at 50% of the phase pick-up to 280 A.

An Extremely Inverse (EI) curve is chosen since it coordinates well with the downstream fuse. The time multiplier for the phase overcurrent is set to establish a coordination time interval (CTI) of at least 0.12 s with respect to the fault level at the downstream 100 A Type ‘T’ fuse. In this case, a CTI of 0.127 s at 2.215 kA.

In addition, an instantaneous high current trip setting is enabled with k = 1.2 x fault 4 (see figure 1) or 2.658 kA which is equivalent to 4.75 times the phase overcurrent pick-up setting. This is configured in ETAP through the ‘Controller’ tab of the recloser. For this controller, the delayed curve is configured as TCC 2.

For the ground overcurrent, the time multiplier is set such that the ground wire is sufficiently protected. In this case, a time multiplier of 0.7. This translates into a CTI of 0.435 s at 1.346 kA with respect to the downstream 100 A Type ‘T’ fuse. An instantaneous high current trip setting is also enabled for ground overcurrent with k = 1.2 x fault 4 (see figure 1) or 1.615 kA which is equivalent to 5.77 times the ground overcurrent pick-up setting.

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The recloser sequence is set to two operations to lockout with the 1st operation using the fast curve (TCC 1). The fast curve is set with the same pick-up level as the delayed curve (TCC 2) but is set to operate instantaneously in order to avoid fuse operations for transient faults. This scheme is particularly known as the ‘Fuse Saving’ scheme.

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The reclosing duration is set at 1 s to allow transient or temporary faults to subside before reclosing. Upon reclosing, the fast curve is locked-out. The 2nd operation uses the delayed curve which is coordinated with the fuse characteristic curve.

Figures 12 and 13 show the phase and ground TCC plots of the recloser fast and delayed curve, 100T, and 65T fuses.

Figures 12 and 13 also indicate the maximum fault levels at each fuse location (fault 4 and 5 in figure 1) in which the coordination time interval with the recloser is evaluated. To do this in ETAP star coordination, open the ‘Star Mode Study Case’ by clicking the ‘Edit Study Case’ icon.

The ‘Study Case’ contains the study preferences and parameters which include the selection of the faulted bus, the standard to be used, the number of protective devices to be considered, calculation tolerance adjustments, and others.

Start by selecting the bus/buses to be faulted. In this case, buses ‘Tap_Sec 3-4’ and ‘Tap_Sec 4-5’.

On the one-line diagram, the faulted buses should be highlighted in red. Click on the ‘Run/Update Short-Circuit kA’. This should update the fault levels of the protective devices connected to the faulted bus.

To verify this, click on any fuse and navigate to the ‘TCC kA’ tab. As shown in figure 17, two values of TCC currents are displayed, ‘TCC Clipping Current’ and ‘TCC Minimum Current’. These values are updated based on the selected ‘Short-Circuit Current’ in the ‘Standard’ tab of the ‘Study Case’. If ‘1/2 Cycle kA’ is selected, the ‘TCC Clipping Current’ values will be updated else if ’30 Cycle kA’ is selected, the ‘TCC Minimum Current’ values will be updated.

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The ‘TCC Clipping Current’ indicates the maximum fault level for the protective device selected. It is shown on the TCC plot as a vertical arrow pointing upwards. The arrow “clips” the device characteristic curves as shown in figures 12 and 13. On the other hand, the ‘TCC Minimum Current’ indicates the minimum fault level for the protective device selected. It is shown on the TCC plot as a vertical arrow pointing downwards.

Circuit Breaker Relay Settings

The maximum load through the 13 kV circuit breaker is 330 A. From this, the current transformer primary rating of 400 A is selected giving a ratio of 400:5 (CTR = 80). The phase overcurrent minimum operating current or pick-up value is set to more than twice the maximum load, at 720 A, to provide security for cold-load transients. For the selected overcurrent relay, an input setting in multiples of the nominal CT secondary rating of 5 A is required. A 720 ampere-primary pick-up with a CTR of 80 translates into a relay pick-up value setting of 1.8. The ground pick-up value is chosen to be 320 A. Thus, a relay pick-up value for ground overcurrent is set to 0.8.

An Extremely Inverse (EI) curve is chosen since it coordinates well with the downstream fuse and recloser curve. The time multiplier for the phase overcurrent is set to establish a coordination time interval (CTI) of at least 0.12 s with respect to the fault level at the downstream 100 A Type ‘T’ fuse and at least 0.2 s with respect to the fault level at the downstream recloser. In this case, a CTI of 0.131 s at 6.131 kA and 0.241 s at 4.478 kA for the fuse and recloser, respectively.

In addition, an instantaneous high current trip setting is enabled with k = 1.2 x fault 2 (see figure 1) or 7.357 kA. This translates to a relay pickup setting of 18.4. The instantaneous element pickup setting is chosen such that it will not overreach the downstream fuse and recloser.

For the ground overcurrent, the time multiplier is set to establish a coordination time interval (CTI) of at least 0.12 s with respect to the fault level at the downstream 100 A Type ‘T’ fuse and at least 0.2 s with respect to the fault level at the downstream recloser. In this case, a CTI of 0.128 s at 5.825 kA and 0.209 s at 3.969 kA for the fuse and recloser, respectively. In addition, an instantaneous high current trip setting is enabled with k = 1.2 x fault 2 (see figure 1) or 6.990kA. This translates to a relay pickup setting of 17.5. Again, the instantaneous element pickup setting is chosen such that it will not overreach the downstream fuse and recloser.

When applying fuse saving at the breaker, consideration should be made on overreaching the downstream recloser. To do this, the pickup value for the fast curve must be set greater than fault 3 (see figure 1). Setting the fast curve pickup at k = 1.2 x fault 3 or 5.374 kA and 4.763 kA for phase and ground, respectively, should be satisfactory in order to prevent overreaching the downstream recloser. However, at this level of pickup, the fuse melting time is 0.0244 s and 0.0309 s, respectively, and with the breaker instantaneous operating time of at least 0.05 s, the fuse will blow before the breaker opens. In this case, fuse saving is not applicable at the breaker.

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Sequence-of-Operation Analysis

ETAP star coordination features a ‘Sequence-of-Operation’ analysis which allows the simulation of a fault at any point on the one-line diagram and assesses the sequence of operation of the protective devices. The parameters for the ‘Sequence-of-Operation’ is accessible through the ‘Edit Study Case’.

To use the ‘Sequence-of-Operation’ analysis, click on the ‘Fault Insertion (PD Sequence-of-Operation)’ icon on the ‘Study Mode’ toolbar as shown in figure 22 and drop on any point of the one-line diagram. The icon changes depending on the ‘Fault Type’ selected.

After inserting a fault, ETAP star coordination will show an animated sequence of protective device operation. The number of protective devices considered and operated will depend on the setting parameters on the ‘Study Case’.

The protective device sequence operation details are available from the ETAP star coordination report manager. Click on the ‘Report Manager’ icon on the ‘Study Mode’ toolbar, navigate to the ‘Summary’, and select the ‘Sequence of Operation’. The report will show the time of operation of the protective devices.

Note: An additional bus was added after the 65T fuse to simulate a downstream fault.

In this example, for a 3-phase bolted fault downstream of the 65T fuse, the recloser acts first with its fast curve issuing a trip signal in 10 ms. The recloser primary contacts open 42 ms after the issued trip signal saving the fuse from tripping. The recloser recloses after 1000 ms. If there was no fuse saving scheme applied, the 65T fuse will have started melting in 240 ms and ultimately clearing the fault in 353 ms. Thus, for a permanent fault downstream of the fuse, the total operation time to completely isolate the fault is 1405 ms (the recloser operates on fast curve and recloses in 1052 ms while the fuse will clear the fault in 353 ms after reclosing).

If the fuse failed to isolate the fault, the recloser will continue to operate on its delayed curve, issuing a trip signal in 1596 ms and ultimately isolating the fault in 1638 ms. The total recloser operation time is 2690 ms. This includes the operation of the fast curve (52 ms), the reclosing time (1000 s) and the operation of the delayed curve (1638 ms).

If the recloser fails, the substation circuit breaker will isolate the fault in 7955 ms (7871 ms for the relay to issue a tripping signal plus 83.3 ms of circuit breaker interrupting time).

References

[1] Blackburn, J. (2014). Protective Relaying Principles and Application, 4th ed. Boca Raton, FL: CRC Press.

[2] IEEE Std 242-2001 [The Buff Book]: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.(2001). S.I.: IEEE.

[3] G. Pradeep Kumar (2006), Power System Protection Design, notes on Power System Protection Training, Visayan Electric Company, Cebu City, Philippines.

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