Coordination is a systematic application of current actuated devices in a power system, which in response to a fault or overload will remove only a minimum amount of equipment from service. The objective is to minimize the equipment damage.
A coordination study provides data useful for selection of instrument transformers, protective relay characteristics and settings, fuse ratings, and other information pertinent to provision of optimum protection and selectivity in coordinating these devices.
Planning and Data Collection.
The following data and initial planning steps are required before a coordination study is started:
• Single-line diagram of the electrical system with details of equipment ratings.
• Load flow data and short-circuit data. The maximum and minimum available short-circuit currents, both for phase and ground faults at each relay location in the system.
• Time-current curves, setting ranges, type of characteristics of the protective devices, instrument transformer connections and ratios.
• Power and voltage ratings and winding connections of all power transformers.
• Normal and emergency switching conditions.
• Transformer impedance data, generator fault decrement curves, equivalent impedances of the interconnected networks, conductor sizes, type and configurations and method of installations.
Coordinating Time Intervals.
When plotting coordination curves, certain time intervals must be maintained between curves of various protective devices in order to ensure the desired selectivity. These intervals take into account the circuit breaker interrupting time, relay overtravel and an arbitrary safety factor to take into account current transformer errors and tolerances in the relay characteristics.
For relayed medium-voltage circuit breakers, interrupting time five cycles, and very inverse and extremely inverse electromagnetic relays, a CTI of 0.4 s is adequate. For solid static relays this can be reduced to 0.3 s because relay overtravel is eliminated.
Relayed circuit breakers with electromagnetic relays can be coordinated with downstream fuses with 0.2 s CTI, which can be reduced to 0.1s with static relays. Coordination between fuses for a time duration of less than 0.01s should not be evaluated on a time-current basis.
Two series connected instantaneous devices will coordinate if the maximum let-through I2t of the downstream device is less than minimum I2t let-through of the upstream devices. Coordination between instantaneous relays without an intervening impedance is generally not possible.
An Example of Protection and Coordination in an Industrial Distribution System. Figure 11 shows the phase overcurrent coordination of the protective devices for 1862.5 kW (2500 hp) motor, 2.4 kV main and feeder breakers, and motor contactor interrupting ratings. The 1862.5 kW (2500 hp) motor is controlled by NEMA 2 motor starter, consisting of a 700 A vacuum contactor and a 650 A type R fuse.
The selected fuse should be the smallest whose minimum melting time characteristics does not cross the motor overload relay for currents less than the adjusted locked rotor current of the motor. The adjusted locked rotor current is taken 10% higher than the actual locked rotor current to account for system voltage variations and manufacturing tolerances.
In order to coordinate with the selected motor fuse, the pick-up settings on overload relays of feeder breaker L serving 2.4 kV control center are set at 1920 A. This exposes the circuit breaker to 160% of its continuous current rating; however, this compromise is acceptable because practically low level of short-circuit currents will not be sustained and each load at the control center has its own overcurrent protection.
The coordination between fuse and 50 MVA interrupting rating of the vacuum contactor for a drop out time of 0.02 cycles is not achieved, and there is a possibility of the contactor clearing a fault current exceeding its interrupting rating.
The remedial measures for this situation can be (1) delaying the opening of the motor contactor, (2) connecting the 1862.5 kW (2500 hp) motor to 4.16 kV system, or (3) devising a special design of the 1862.5 kW (2500 hp) motor to reduce the locked rotor current permitting a lower fuse size.
The example illustrates the judgment that a protection engineer should make in accepting compromises in a given situation for arriving at an acceptable engineering solution.
Recent trends in protective relaying are being dictated by advancements in electronics, microprocessor technology, programming, and packaging. It would have been impossible to detect an impeding bearing failure in a motor using electromagnetic devices; however, neural network techniques to characterize the current spectra associated with a normal state of a motor and load makes the detection possible by monitoring the changes in the bearing frequencies as reflected in the current spectra.
Developments in high-impedance fault detection (HIFDs) fault localization systems, charge comparison type of current deferential relaying and adaptive relaying are further examples. More knowledge-based systems and new algorithms will be applied to protective relaying, coordination, service restoration, and remedial control actions.
Multifunction microprocessor-based relays make it possible to integrate a number of protective functions, metering data, fault location, remote communication, and data logging in a single modular package unit.
As an example, most of the generator protective functions are available in a single unit with added facilities of self-diagnostics, communications, and fault data capture.
Phase overcurrent device coordination: 1862.5 kW (2500 hp) motor, 2.4 kV main and feeder breakers, and fused motor contactor. 1:Motor full load current = 545 A; 2: Motor relay pickup = 646 A; 3: Motor thermal damage curve; 4: Motor locked rotor current; 5: Motor adjusted locked rotor current; 6: 650 A motor fuse characteristics; 7: dropout time variations, vacuum contactors; 8: dropout time air-break contactors; 9: Inrush current of the motor control center (largest motor); 10: breaker K overcurrent relays pickup = 2800 A; 11: breaker L overcurrent relays pickup = 1920 A; 12: three-phase sym. Short circuit current after 6 cycles = 27.27 kA, with in-plant generator only in service = 11.74 kA; 13: Interrupting kiloampere vacuum contactor, 14: Interrupting kiloampere air-break contactor; 15: transformer let-through current = 57.20 kA asym.