Protective Device Coordination — ETAP
Protective devices such as fuses and circuit breakers are playing a big rule in power system protection. When a fault occurs on the electrical system short-circuit/overloading the fault current must be interrupted quickly.
Coordination study is required to coordinate upstream with downstream branches to minimize interruption of electrical power in other areas of the network and allow only the protective device nearest the fault to open. With that, faulted branch would affect only that area where the failure occurs to achieve service continuity.
The objectives of overcurrent coordination are to determine the characteristics, ratings, and settings of overcurrent protective devices that minimize equipment damage and interrupt short circuits.
If there is a fault in downstream the nearest CB tripped first while unsuccessful coordination the upstream CB tripped before the downstream leading to de-energize the entire system.
Perform Coordination Study
This article explains how to perform protective device coordination using ETAP 12.6. So as always, the first thing to do is modeling your system/network as a single line diagram then, set all the parameters for each component in the network.
We will select protective device coordination analysis mode in the mode toolbar.
Once enter coordination analysis mode, the study toolbar automatically appears on the right-hand side of the screen.
- Create star view allows to graphically present the device’s behavior/response to a different level of overcurrent, the shadow curve present tripping point across different characteristics(long time, short time, instantaneous time). In star view characteristics curves for the protective devices can be graphically adjusted, just by clicking and dragging as shown below.
To shows complete information related to the device just right click on the label tag associated with the curve and check the Settings options.
- protect zone this option shows the protective devices within the network on zones and divided to portions for example utility, transformer, load zone and create a star view for each zone separately.
- Sequence Viewer clicking the Sequence Viewer button, displays the Sequence-of-Operation Events dialog box that provides a tabulated sequence summary list of actions for the applicable protective devices.
- Fault insertion allows us to simulate, show devices tripping sequence in case of fault happened.
Transformer Damage Curve: Fuse/breaker is used to protect the transformer and cable from faults. To achieve this, the fuse/breaker curve should be to the right of the transformer inrush point and to the left of the cable & transformer damage curve in order to sure that the fuse/breaker will operate before reaching the damage curve.
The important factors as per IEEE std. to be considered when coordinating transformer protective relays are:
- Transformer voltage, kilovoltamperes, and impedance ratings
- Primary and secondary winding connections
- Connected load
- Transformer magnetizing inrush current
- Transformer thermal and mechanical protection curves
- Short-circuit current available on both the primary and secondary
Cable Damage Curve: in terms of cable protection, as mentioned before, breaker curve has to the left of the cable damage curve to sure that the breaker will protect our cable.
So, let’s say we want to improve the following poor coordination system. we have to adjust the breakers setting where the nearest to the fault point will operate first.
To solve this issue characteristics/breakers setting should be checked and adjusted correctly. To do that, double click on the Breakers in your SLD, go to the Trip device page, the following characteristics (long time, short time, instantaneous time) will show up.
by comparing short time (ST) of both CBs, main feeder ST is 1875A and 2400A for branch (which less than Branch ST). Result of this setting, the upstream CB tripped before the downstream and leading to shutdown the entire system.
Characteristics adjustment need to study network and take the load type, application into account to achieve successful coordination. The downstream short time must be less then upstream to trip first and minimize the effected branch feeders.
We learned in this tutorial:
- Importance of preforming coordination study
- What is the time current curve (TCC)
- How to preform coordination study using ETAP
- Coordinating transformer & cables protective devices
- How to generate star view and adjusted it graphically or from Trip device page
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- ETAP help, ETAP 12.6.0 software.
- IEEE 242 Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.
- National electric code (NEC),articles 240.12,708.54.