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Home / Technical Articles / Protection coordination practices in distribution systems with distributed generation

Protection coordination study

Generally, in most of the countries that cover large area, there are three types of distribution systems: urban, suburban, and rural. Each type of them has its own features regarding the length of the backbone, types of protection devices used, types of laterals, load density, and voltage level.

Protection coordination practices in distribution systems with distributed generation
Protection coordination practices in distribution systems with distributed generation

This technical article is based Canadian power distribution system. Protection coordination is performed for urban, suburban, and rural distribution systems with distributed generation (DG) installed.

Table of contents:

  1. Current protection practices
    1. Protection devices
    2. Coordination of protection devices
  2. Urban distribution system
    1. Protection coordination study
  3. Suburban distribution system
    1. Protection coordination study
  4. Rural distribution system
    1. Protection coordination study

1. Current Protection Practices

1.1 Protection Devices

There are several protection devices used in the protection of the different types of distribution systems. A List of the most common current protection devices is shown below:

  1. Instantaneous phase overcurrent relay (50P): Used at the main feeder head end.
  2. Timed phase overcurrent relay (51P): Also used at the main feeder head end.
  3. Timed ground overcurrent relay (51G): Also used at the main feeder head end.
  4. Timed negative sequence overcurrent relay for phase-phase faults (46): Also used at the main feeder head end.
  5. Impedance relay (21)
  6. Reclosers (70)
  7. Current limiting fuses
  8. Differential protection for station bus only
The selection of the relay settings differs from utility to utility. But there are general rules are used to select the setting of each relay. For example, the pickup setting of the timed phase overcurrent relay is selected to be secure under normal loading condition and to be dependable under over loading or short circuit conditions.

Some utilities prefer to use lower pickup setting for the relay to ensure dependability under overloading condition and other utilities prefer to use slightly higher pickup setting of the relay to ensure reliability in case of temporary overloading.

The time dial setting (TDS) and the curve type of the relay is selected according to coordination criteria of utility.

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1.2 Coordination of Protection Devices

Almost all electric utilities use the concept of overlapped zones in the coordination of the overcurrent protection devices. This means that each zone should have its own primary protection and a backup protection facility that operates in case of primary protection failure.

In most of the cases, the backup protection is the next upstream protection device to the protected zone. Backup protection should operate only when the primary protection stuck and didn’t operate.

Figure 1 shows a typical radial distribution system.

For the fault shown, the fuse F1 should respond very fast to this fault, as this fuse is the primary protective device of this zone. The relay-circuit breaker set (R1-B1) are considered the backup protection for the marked zone that should operate in case of F1 failure. This implies that operating time of R1 should be larger than that of F1 for any fault in the marked zone.

A typical radial distribution feeder
Figure 1 – A typical radial distribution feeder

There are some issues that should be considered during the preparation of any coordination study:


Issue №1 – Mis-coordination problem

This means that the coordination study should avoid any miscoordination between the protective devices. For example, for the system shown in Figure 1, the fuse F1 (primary protection) should operate before the relay R1 (backup protection) for any fault on the protected zone.

This ensures the selectivity.


Issue №2 – Fuse saving strategy

This strategy saves the fuse from blowing during temporary faults. This strategy is done using a recloser that opens the circuit and recloses it again very fast to clear the temporary fault and save the fuse.

This is done in rural and suburban distribution systems.


Issue №3 – Sensitivity of the main head end relay

This means that the main feeder head end relay should sense any fault in the main feeder under any condition. The addition of the distribution generation (DG), for example, to the distribution system might reduce the fault current level drawn from the main substation. This will in turn affect the operation of the substation breaker or recloser especially on their ability to “see” the fault.

This will be highly dependant on the type, size and location of the distribution generation (DG). The main relay of the feeder should be designed to overcome such problems.

Recommended reading:

Electricity distribution systems, substations & integration of distributed generation


Issue №4 – Bi-directionality

This issue is obvious for radial feeders that are fed from the same substation. Protection devices on one feeder may respond to faults in the other feeder due to back feed especially if distribution generation (DG) is installed in the healthy feeder.

Also, this issue should be considered during the design stage of the protection system.


Issue №5 – Overvoltage considerations

Overvolatges may occur during faults if the system is ungrounded. This problem is obvious if a DG is installed and interfaced via ungrounded transformer with the main feeder.

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2. Urban Distribution System

Figure 2 shows an example of a single line diagram of the urban distribution system. The urban system, as shown in Figure 2, has been designed in CYMDIST environment.

The utility substation is modeled by an equivalent source behind impedance. The station is rated at 100 MVA with a station bus feeding twelve feeders. The 12.5 kV substation bus is a split-bus type with a normally open tie-breaker. A 1-Ohm series current-limiting reactor is used to limit the short circuit current flowing through the system.

Due to the similarity in all feeders, the remaining five feeders on each bus are modeled as a lumped load connected at the main station bus with a total power demand of 42 MVA and an overall power factor of 0.95 lagging.

A 10 MVAR delta connected capacitor bank is interconnected at the main station bus for reactive power compensation.

Single line diagram for the urban distribution system
Figure 2 – Single line diagram for the urban distribution system

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2.1 Urban Distribution System Protection Coordination Study

Short circuit analysis was performed on the urban distribution system given in Figure 2 above. The short circuit currents vary according to system configuration. When conducting the protective devices coordination study, it is mandatory to consider the configuration, which results in maximum short circuit currents to make sure that the protective devices are rated to withstand the worst case scenario fault currents.

The case where the tie-breaker is closed was considered for protective devices coordination.

In addition, if protection devices coordinate for high fault currents (with the tie breaker is closed) then the protective devices will be coordinated for lower faults current, i.e. when the tie breaker is open. For this reason, the case where the tie-breaker is closed was considered for protective devices coordination. A 1-Ohm series current-limiting reactor was used to reduce the short circuit current levels.

The different feeder paths are examined to assure proper coordination between various protective devices. In order to ensure proper coordination, different circuit paths are studied.

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Path 1: – From Station Bus ➜ Bus 7

This circuit starts from the main substation and extends towards the last bus on the system (Bus 7). This path includes the main feeder relay as well as the main overhead lateral fuse and other individual fuses of each connected load on the lateral.

Figure 3 (see below) shows the one line diagram as well as the protective device coordination chart. As seen in Figure 3, the S&C SMU K fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values and at the same time coordinates with the transformer damage curve.

The S&C SMU 20 K fuse was designed to operate as a backup for the S&C SMU K fuse where the coordination time interval is approximately 0.1 seconds. The main feeder relay acts as a backup for the S&C SMU 20 K fuse where the coordination time interval is 0.9 seconds.

In addition, the ground settings of the main feeder relay have been coordinated with the S&C SMU 20 K fuse and the coordination time interval is approximately 0.3 seconds.

Furthermore, Figure 9 indicates that the ground-over current coordination is fulfilled.

Urban system coordination chart - Path 1: station Bus ➜ Bus 7
Figure 3 – Urban system coordination chart – Path 1: station Bus ➜ Bus 7

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Path 2: – From Station Bus ➜ Bus 5 ➜ Bus 6

This circuit starts from the main substation and extends towards bus 5, which is connected to Bus 6 (underground lateral). This path includes the main feeder relay as well as the main underground lateral fuse and the individual fuses of each connected load on the lateral.

Figure 4 (see below) shows the one line diagram as well as the protective device coordination chart.

As seen from Figure 4, the S&C SMU 40 fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values and at the same time coordinates with the transformer damage curve. The S&C SMU K fuse was designed to operate as a backup for the S&C SMU 40 fuse where the coordination time interval is approximately 0.1 seconds. The main feeder relay acts as a backup for the S&C SMU K fuse where the coordination time interval is 0.6 seconds.

In addition, the ground settings of the main feeder relay have been coordinated with the S&C SMU K fuse and the coordination time interval is approximately 0.4 seconds.

Urban system coordination chart - Path 2: station Bus ➜ Bus 5 ➜ Bus 6
Figure 4 – Urban system coordination chart – Path 2: station Bus ➜ Bus 5 ➜ Bus 6

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Path 3: – From Station Bus ➜ Bus 3 ➜ Bus 4

This circuit starts from the main substation and extends towards bus 3, which is connected, to Bus 4. This path includes the main feeder relay as well as the main transformer fuse. Figure 5 (see below) shows the one line diagram as well as the protective device coordination chart.

As seen in Figure 5, the C-H DBU fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values and at the same time coordinates with the transformer damage curve. The main feeder relay acts as a backup for the C-H DBU fuse where the coordination time interval is approximately 0.4 seconds.

In addition, the ground settings of the main feeder relay have been coordinated with the C-H DBU K fuse and the coordination time interval is approximately 0.4 seconds.

Urban system coordination chart - Path 3: station Bus ➜ Bus 3 ➜ Bus 4
Figure 5 – Urban system coordination chart – Path 3: station Bus ➜ Bus 3 ➜ Bus 4

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Path 4: From Station Bus ➜ Bus 1 ➜ Bus 2

This circuit starts from the main substation and extends towards bus 3, which is connected, to Bus 4 (motor and spot load). This path includes the main feeder relay as well as the main transformer fuse.

Figure 6 (see below) shows the one line diagram as well as the protective device coordination chart.

As seen in Figure 6, the S&C SMU 20 fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values, motor starting currents and at the same time coordinates with the transformer damage curve. The main feeder relay acts as a backup for the S&C SMU 20 fuse where the coordination time interval is approximately 0.4 seconds.

In addition, the ground settings of the main feeder relay have been coordinated with the S&C SMU 20 fuse and the coordination time interval is approximately 0.4 seconds.

Urban system coordination chart - Path 4: station Bus ➜ Bus 1 ➜ Bus 2
Figure 6 – Urban system coordination chart – Path 4: station Bus ➜ Bus 1 ➜ Bus 2

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3. Suburban Distribution System

Figure 7 shows an example of the single line diagram of the urban distribution network. The suburban system, as shown in Figure 7, has been designed also in CYMDIST environment. The utility was modeled by an equivalent source behind impedance. The substation is a double-ended type with one transformer on each side, each rated at 100 MVA. The rated bus voltage is 24.9kV.

Each bus feeds six feeders and the tie-breaker is normally closed. Normally closed circuit breaker would allow feeding from separate transformers. However, a normally closed circuit breaker leads to a considerable increase in the short circuit level. For this reason a 2-Ohm aircore fault-current limiting series reactor is introduced at feeder head end.

The presence of the 2-Ohm series reactor remarkably decreases the symmetrical short circuit level from around 38 kA to 6 kA.

On the other hand, the introduction of the series reactor would introduce large voltage drop. Therefore, the sending end voltage should increase with equivalent amount to insure that the voltage drop at the end of the main feeder and at the end of all laterals is within the acceptable limit (5%).

Therefore, the substation voltage is adjusted at 1.024 p.u. Due to the similarity in all feeders, the remaining five feeders on each bus are modeled as a lumped load connected at the main station bus with a total power demand of 84 MVA and an overall power factor of 0.95 lagging.

A 20 MVAR delta connected capacitor bank is interconnected at the main station bus for reactive power compensation.

Single line diagram of the suburban distribution system
Figure 7 – Single line diagram of the suburban distribution system

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3.1 Suburban Distribution System Protection Coordination

Short circuit analysis was performed on the suburban distribution system given in Figure 7 above. Protective device coordination is based on the assumption that the station tie-breaker is closed and a 2-Ohm, 400 A, series air core reactor is placed on feeder end head to reduce the short circuit level under 8 KA.

The maximum short circuit current with series reactor installed is around 6 kA for three phase bolted fault at the feeder head end. The different feeder paths are examined to assure proper coordination between the various protective devices.

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Path 1: – From Station Bus ➜ 1 MVA Commercial Load

This circuit starts from the main substation and extends towards the last bus on the system (Bus 5). This path includes the main feeder relays, the recloser, the main underground lateral fuse (F6), and the 1 MVA transformer.

Figure 8 (see below) shows the one line diagram as well as the protective device coordination chart.

As seen from Figure 8, Kearney 40T fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values and at the same time coordinates with the transformer damage curve. The T fuse-family is more preferred than the K family in this application, this because the slower response of the T family allows better coordination with the fast curve of the recloser.

The fault current setting of the feeder head end instantaneous relay is 1500 A. The recloser is coordinated with the fuse for fuse saving up to 1600 A.

Overcurrent relay with delayed time setting and pickup current of 600 A is adjusted to allow for two recloser operations before it clears the fault.

Suburban system coordination chart - Path 1: station Bus ➜ 1 MVA commercial loads
Figure 8 – Suburban system coordination chart – Path 1: station Bus ➜ 1 MVA commercial loads

So for fault on laterals, which is almost double lateral peak load current and under 560 A, the Kearney 40T fuse will clear the fault. For faults on laterals with short circuit current levels in the range 560 A to 1500 A, the recloser will operate once to allow for fuse saving for non-permanents fault. If the fault persists after the first operation of the recloser, the Kearney 40T fuse will clear the fault.

For faults with short circuit current equal to or larger than 1500 A, the instantaneous relay would operate to interrupt the fault. The overcurrent relay time delay is adjusted to allow for two recloser operations during fault before it interrupts the fault.

Using the same approach, the ground overcurrent coordination study can be conducted for path 1 as shown in Figure 9 below.

Suburban system coordination chart - Path 1: station Bus ➜ 1 MVA commercial loads – ground coordination
Figure 9 – Suburban system coordination chart – Path 1: station Bus ➜ 1 MVA commercial loads – ground coordination

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Path 2: – From Station Bus ➜ Bus 4 (Motor Bus)

This circuit starts from the main substation and extends towards (bus 4) which is then connected to the1 MVA induction motor load. This path includes the main feeder relay, the recloser, the main lateral fuse (F5), and the 1 MVA transformer and 1 MVA motor.

Figure 10 (see below) shows the one line diagram as well as the protective device coordination chart.

As seen from Figure 10, the Kearney 40T fuse was chosen such that it will not operate for transformer inrush current, induction motor starting current values and at the same time coordinates with the transformer damage curve. The Kearney 40T fuse would clear faults in laterals for any short circuit current levels ranging from almost the double peak load value up to 560 A.

For faults on laterals with short circuit current levels in the range 560 A to 1500 A, the recloser will operate once to allow for fuse saving for non-permanent faults. If the fault persists after the first operation of the recloser, the Kearney 40T fuse will clear it. For faults with short circuit current equal to or larger than 1500 A, the instantaneous relay would operate to interrupt the fault.

The overcurrent relay time delay is adjusted to allow for two recloser operations during fault before it interrupts the fault.

Suburban system coordination chart - Path 2: station Bus ➜ Bus 4 (motor bus)
Figure 10 – Suburban system coordination chart – Path 2: station Bus ➜ Bus 4 (motor bus)

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Path 3: – From Station Bus ➜ Bus 2

This circuit starts from the main substation and extends towards bus 2 which is then connected to residential load areas via underground cable to 21 single phase transformers rated 100 KVA each. This path includes the main feeder relay, the recloser, the main lateral fuse, and the 100 KVA transformer fuse.

Figure 11 shows the one line diagram as well as the protective device coordination chart. Similar to previous paths, protection devices coordinate.

Suburban system coordination chart - Path 3: station Bus ➜ Bus 2
Figure 11 – Suburban system coordination chart – Path 3: station Bus ➜ Bus 2

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Path 4: – From Station Bus to 3

Coordination along this path is identical to Path 3.

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4. Rural Distribution System

A typical Canadian rural distribution feeder has been utilized as a rural benchmark distribution system. Figure 12 shows the one line diagram of the urban distribution network under study.

The rural system, as shown in Figure 12, has been modeled also in CYMDIST environment.

The utility was designed by an equivalent source behind impedance. The substation rating is 20 MVA, and the feeder rated voltage is 27.6 kV. A regulating station is located around 12 km along the main feeder. The regulating station setting is adjusted to boost the voltage by 2.5%.

Single line diagram of the rural system
Figure 12 – Single line diagram of the rural system

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4.1 Rural Distribution System Protection Coordination

Short circuit analysis was performed on the rural distribution system given in Figure 12. The maximum short circuit current was around 19 kA for three phase bolted fault at the feeder head end.

The different feeder paths are examined to assure proper coordination between the various protective devices.

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Path 1: – From Station Bus ➜ Load M13

This circuit starts from the main substation and extends towards load M13. This path includes the main feeder relay, the recloser, the main lateral fuse (F1), and the transformer (T3). Figure 13 (see below) shows the one line diagram as well as the protective device coordination chart for this path.

As seen in Figure 13, the 40 K fuse (primary transformer fuse) was chosen such that it will not operate for transformer inrush current values and at the same time coordinates with the transformer damage curve. Also, the majority of the through-fault curve of transformer (T3) (including the damage point, which is at the end of the curve) is located over the clearing curve of the fuse.

This guarantees safe operation of the transformer under through-fault conditions.

Furthermore, the fast curve of the recloser and the fuse coordinate up to 2625 A for the phase protection curve. This band-limited coordination is sufficient for the maximum short circuit current at the fuse location 2128.

Furthermore, Figure 13 indicates that the upstream relay totally coordinates with the slow curve of the recloser.

Rural system coordination chart - Path 1: station Bus ➜ Load M13
Figure 13 – Rural system coordination chart – Path 1: station Bus ➜ Load M13

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Path 2: – From Station Bus ➜ Load M21

This circuit starts from the main substation and extends towards load M21. This path includes the main feeder relay, the recloser, and the main lateral fuse (F5). Figure 14 (see below) shows the one line diagram as well as the protective device coordination chart for this path. For this single-phase lateral, the maximum short circuit current is around 1188 A.

The band-limited coordination between the recloser and the fuse is very sufficient for the maximum short circuit at the fuse location in this case.

Furthermore, the upstream relay totally coordinates with the slow curve of the recloser.

Rural system coordination chart - Path 2: station Bus ➜ Load M21
Figure 14 – Rural system coordination chart – Path 2: station Bus ➜ Load M21

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Path 3: – From Station Bus ➜ Load M25

Coordination along this path is similar to Path 2 with 1003 A short circuit current at the fuse location.

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Path 4: – From Station Bus ➜ Load M14

Coordination along this path is similar to Path 2 with 954 A short circuit current at the fuse location.

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Path 5: – From Station Bus ➜ Load M16

Coordination along this path is similar to Path 2 with 893 A short circuit current at the fuse location.

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Source: Protection coordination planning with distributed generation by CETC Varennes – Energy Technology and Programs Sector (Dr. Tarek K. Abdel-Galil, Ahmed E.B. Abu-Elanien, Eng., Dr. Ehab F. El-Saadany, Dr. Adly Girgis, Yasser A.-R. I. Mohamed, Eng., Dr. Magdy M. A. Salama, Dr. Hatem H. M. Zeineldin at Qualsys Engco. Inc.)

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

7 Comments


  1. Mohamed Omar Morad
    Jul 13, 2020

    This is a very good article and full of descriptions and very helpful.
    I recommend every Senior dialectical Engineer to read it carefully. Regards


  2. Mohamed Omar Morad
    Jul 13, 2020

    This is a very good article and full of descriptions and very helpful. I recommend every Senior dialectical Engineer to read it carefully. Regards


  3. sana
    Jul 12, 2020

    Can u send in pdf


  4. Wan Farid
    Jul 11, 2020

    Hi brother, could you please email me this article in pdf format.

    Farid,


  5. Engr. Eval Asikong
    Jul 09, 2020

    Can you please send this to my email, and also articles on Improving voltage control for distribution system network with distributed energy resources.


  6. irfan ullah
    Jul 09, 2020

    hi brother could you please email me this article in pdf format…

    regards

    IRFAN ULLAH MARWAT


  7. Ibrahim Mohamed Ibrahim
    Jul 08, 2020

    Kindly send me articles related to protection coordination

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