Continued over currents increase the resistance heating (I2R) of conductors and can decrease cable insulation life and cause failures. Conductors are normally protected by overcurrent protective devices, with the pickup settings based on the cable ampacity. The NEC provides rules for the protection of conductors. These rules are generally based on the long-time pickup rating of the protective device and the normal full-load rating (or ampacity) of the conductor.
The conductor short-time heating limits, based on short-circuit currents or on allowable emergency overload currents, are additional points that should be plotted to ensure that the protective devices provide adequate protection for the conductor.
In coordinating system protection, the conductor should be able to withstand the maximum through-fault current for a time equivalent to the tripping time of the upstream protective device. Another factor in protecting the circuit cable is the maximum short-circuit current available at the extremity of the cable circuit. The conductor insulation should not be damaged by the high conductor temperature resulting from current flowing to a fault beyond the cable termination.
As a guide in preventing insulation damage, curves of conductor size and short circuit current based on temperatures that damage insulation are available from cable manufacturers. In coordinating system protection, the cable should be able to withstand the maximum through short-circuit current for a time equivalent to the tripping time of the primary relay protection or total clearing time of the fuse.
Many times this requirement determines the minimum conductor size applicable to a particular power system. If it is not possible to select a device that will protect the cable insulation, it is recommended that a conductor large enough to carry the current without insulation damage be used.
Overcurrent Protection of Motor
The overcurrent protection of motors includes both overload and short-circuit protection, as required by the NEC. However, additional factors should be considered when applying overcurrent protective devices to motors. These factors include locked-rotor current, acceleration time, and safe stall time. Complications arise from the fact that the same devices that protect the motor from thermal damage must also allow the motor to start.
Motor protection points that are generally plotted on the overcurrent coordination curve include root-mean-square (rms) asymmetrical starting current, locked-rotor current , acceleration time, allowable stall time, and full-load current. The motor starting curve normally shows the symmetrical starting current, but the initial starting current is asymmetrical with the maximum occurring at a 0.5 cycle. Peak-current-sensing protective devices are sensitive to this current.
Therefore, engineering judgment should be used when choosing the protective device type and settings to account for the asymmetrical current during startup to prevent false tripping of the short-circuit protective device. A typical rms asymmetrical starting inrush current would be about 1.76 times the symmetrical locked-rotor current. The point on the voltage waveform influences this inrush factor on each phase when the contactor closes, the X/R ratio of the power system, and the X/R ratio of the motor.
This inrush factor could be as high as 2 to 3 times for stiff power systems for large high-efficiency motors. The locked-rotor current should be obtained from manufacturers data. The acceleration time of the motor, based on the normal means used to start the motor and driven load, should be plotted. The motor acceleration time can be obtained from the motor manufacturer. The motor permissive stall time, which may be given as both hot and cold stall times, should be plotted as well. The overcurrent protection should give enough time delay to allow the motor to start, but not so much that the operating time at locked-rotor current is above the permissive stall times.
If the acceleration time is above the stall time, special relaying considerations may be required. The motor full-load current should be plotted, and a benchmark should also be plotted for the maximum permitted overcurrent device setting for overload protection based on the NEC. Motor overload and short-circuit protection are often provided by a combination of devices. On low-voltage systems, this combination is usually an overload relay with a current-limiting fuse or an overload relay with a low-voltage circuit breaker. The overload relay should be selected (or set) based on the full-load current and service factor of the motor.
The fuse or circuit breaker should be selected or set to protect the motor circuit during short circuits, but should not interrupt normal starting currents. As a result, the time-current characteristic (TCC)of the combination device must fall below and to the left of the motor thermal limit curve and fall above and to the right of the motor starting curve.
Overcurrent Protection of Transformer
Transformers are subject not only to insulation damage from prolonged overloads and short circuits, but also to mechanical damage from the tremendous stresses experienced during a fault. Because of their importance in the plant electrical distribution system, the transformer should be not only well protected, but also secure from inadvertent trips due to faults elsewhere in the system. The transformers be protected from overcurrent with the use of protective devices on the primary and secondary. The important factors to be considered when coordinating transformer protective relays are
- Transformer voltage, kilovolt amperes, 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
IEEE Standard C57.109-1993 recommends protection based on the size of the transformer and the number of estimated through-faults that the transformer is expected to encounter. The through-fault protection curves contained in IEEE Standard C57.109-1993 should be used as the basis for setting transformer overcurrent protective relays. In addition, the maximum inrush current, usually assumed to be at 0.1 s and ranging from 8 to 12 times the transformer self-cooled rated current, should be plotted to ensure the protective relays do not trip on transformer energization.
A final protection point is the maximum permitted overcurrent setting for overload and short circuit protection based on NEC rules. The overcurrent device on the transformer primary should provide protection against thermal and mechanical damage; yet allow the normal connected load to flow.
Overcurrent Protection of Generator
Generators are complex and require a variety of protective devices. Overcurrent devices usually provide backup protection to other generator relays, such as differential.
Because the short-circuit current available from a generator decreases over time, the use of standard overcurrent relays is not practical. The relay pickup setting should be low enough to trip in response to the minimum sustainable generator contribution (i.e., synchronous current E/Xd), but should not trip unnecessarily due to normal overloads.
To accommodate these requirements, a voltage-restrained or voltage-controlled overcurrent relay (Device 51V) is used. This device allows the relay to differentiate between system faults and generator faults. When a fault occurs near the generator, the depressed voltage allows the voltage-restrained relay to become more sensitive. The voltage-controlled relay operates as a simple switched overcurrent relay. Both of these relays require coordination with local and upstream protective devices.
The voltage- restrained overcurrent relay should be selective over its entire range of operation. In addition, many small, low-voltage generators are protected only by MCCBs or power circuit breakers with direct acting trip devices. These protective devices may provide fault protection for the generator, but probably will not provide any backup protection for system faults, due to the generator decrement.