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
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.
Figure 1.
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.
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