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Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

Co-Ordination Studies

Each electrical distribution system consists of electrical apparatus which primarily functions is to supply electrical power from the utility power system to the loads or motor centers with the required voltage level and energy demand. A primary consideration of the design is that the chosen equipment, such as transformers, cables, etc. is capable of providing the energy demand, as well as to withstand fault conditions.

The power distribution system and its components can be protected by various types of fuses, relays, low voltage circuit breakers, trip units, etc. There are three basic considerations during a protection device system design:

* that system has to be safe for the personnel
* that system has to be capable of fully protecting electrical power equipment such power cables, transformer, motors, etc.
* that system has to be selective (co-ordinated)

A short circuit study should be performed to provide short circuit fault levels on each bus in the system. The data needed for this study include ratings and impedance of transformers, cables, motors, etc., as well as the power utility contribution on every connection point within the distribution system. The interrupting ratings of all of the breakers and fuses should be greater than the calculated fault levels available at the point of application. If some of the circuit breakers or fuses have an interrupting rating level less than fault level, then the changing of that apparatus should be considered.

To obtain proper protection, the settings of the protection devices must be so selected to first, satisfactorily protect electrical equipment from overloads (<250% full load) and second, interrupt short circuit (500%-2000% fall load) as rapidly as possible.

The objective of coordination study is to coordinate system so that the protective device closest to the fault on the power-source side has the first chance to operate; but each preceding protective device upstream should be capable, within its designed settings of current and time, to effect the isolation if the fault persists. When this is accomplished, the protective devices are termed selectively coordinated.

Knowing the short circuit fault levels, the electrical equipment ratings and the protective devices settings, the time-current characteristic of the protective devices and the electrical equipment withstand damage characteristic are plotted. By changing the ratings and adjusting the settings of the protective devices, greater protection of the distribution system and coordination of the protective devices can be obtained. To accomplish this, the tripping characteristics of the overcurrent devices should not overlap but should maintain a minimum time interval between them for all current values.

To assure complete coordination, the respective time-current trip characteristics should be simultaneously displayed on a log-log curve to determine adequate time-delay separation between operation of the various devices. This display of characteristics can also include devices at a different voltage level of the system, provided proper consideration of the relative voltage levels is made. In this manner, those fuses on the primary of transformers and high voltage breakers which are controlled by relays can be coordinated with low-voltage breakers. If the ground fault current protection is incorporated in the system, it should be also included in the time-current coordination plots.

Once, the coordination of protective devices are performed, that is not guarantee that the system will remain coordinated forever. System changes and additions plus power source changes frequently modify the protection requirements, leaving some of the protective devices unable to properly interrupt the available fault currents. Consequently, periodic study of protective-device settings is as important in preventing power outages as is periodic maintenance of the distribution system.

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers