Protective relays Thermal

Static thermal relay (discrete ICs or microprocessor based) For medium and large motors, 300 h.p. and above, this type of protective relay should be preferred to achieve optimum utilization of the motor s capacity. Consider the available setting ranges in the vicinity of... 

Overload devices in current use are typically thermal overload relays to BS 4941, motor starters for voltages up to and including 1000 V A.C. and 1200 V D.C. or BS 142 electrical protection relays. Relays to BS 4941 generally provide overload and single-phasing protection. Those complying with BS 142 are also frequently fitted with instantaneous earth fault and over-current trips. 

A thermowell is required to protect the thermal element of a thermometer or temperature transmitter thermobulb from corrosion and erosion, to give it adequate support, and to permit its removal without interrupting the process. The use of the thermowell will unavoidably introduce a temperature time lag to the changes in the process temperature and response to the temperature relayed to the instrument. This is caused by the transmission of heat through the thickness of the metal well and the inevitable dead air space between the well and bulb. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

The thermal withstand time for the neutral earthing resistors are usually specified as 10 seconds for the duration of the fault current. This allows adequate time for main and back-up protection relays to operate and clear the fault. The standby earth fault relay (51 G) time-current characteristic must be chosen so that its Pt curve is lower than that for the neutral earth resistor and the connecting cables. (The curve is derived directly from the I-t data, and not by integrating the curve.)... 

Overload Protection Overload relays lor protecting motor insulation against excessive temperature are located either in the motor control or in the motor itsell. The most common method is to use thermal overciirrent relays in the starter. These relays have heating characteristics similar to those ol the motor which they are intended to protect. Either motor current or a current proportional to motorline current passes through the relays so that relay heating is comparable to motor heating. 

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

With the help of this equation the thermal curve of a machine can be drawn on a log-log graph for a known r, I versus /j//, for different conditions of motor heating prior to a trip (Figure 3.12). The relay can be set for the most appropriate thermal curve, after assessing the motor s actual operating conditions and hence achieving a true thermal replica protection. 

Figure 3.13 Thermal curves to set the relay for over-current protection corresponding to different operating temperatures...

Protection against overloading This can be achieved by an overcurrent relay. The basic requirement of this relay is its selectivity and ability to discriminate between normal and abnormal operating conditions. Three types of such relays are in use thermal, electromagnetic and static. Thermal relays are employed for motors of up to medium size and electromagnetic and static relays for large LT and all HT motors, as discussed in Section 12.5. 

Characteristics of a bi-metallic thermal relay The thermal characteristics are almost the same as those of an induction motor. This makes them suitable for protecting a motor by making a judicious choice of the right range for the required duty. (See Figure 12.11 for a typical thermal overload relay and Figure 12.13 for its thermal characteristics.) Ambient temperature com-... 

Figure 12.16 Supplementing a thermal relay with an IDMT relay for complete motor protection...

Overcurrent protection. To provide a thermal replica protection, the relay is set according to motor s heating and cooling (/ - 1) curves supplied by the motor manufacturer. If these curves are not available, they can be established with the help of motor heating and cooling time constants, as in equations (3.2) and (3.4). A brief procedure to establish the motor thermal curves when they are not available is explained in Section 3.6. 

Thus, besides voltage unbalance it can also detect an inter-turn fault, which leads to a current unbalance. A small amount of unbalance is already detected by the thermal element of the overcurrent protection but a severe unbalance, such as during a single phasing, would require quicker protection and hence, this protection. The relay may be set for an of around 3% or so. 

For small motors with a number of brands and varying thermal characteristics the above may not be practical. Moreover, to arrange the thermal curves for each relay and motor and then match them individually for closer protection may also not be practical. The practice adopted... 

They will make and break, without damage, all currents falling even outside the protected zone of a thermal overcurrenl relay or the built-in overcurrent (o/c) and short circuit (s/c) relea,ses of a breaker, but within the protected region of the HRC fuses, as illustrated by the hatched portion, of the overcurrenl and short-circuit,... 

Harmonic currents influence the operation of protective devices. Fuses and motor thermal overload devices are prone to nuisance operation when subjected to nonlinear currents. This factor should be given due consideration when sizing protective devices for use in a harmonic environment. Electromechanical relays are also affected by harmonics. Depending on the design, an electromechanical relay may operate faster or slower than the expected times for operation at the fundamental frequency alone. Such factors should be carefully considered prior to placing the relays in service. 

Electrical components which could ignite a hazardous atmosphere either by sparking, arcing or by thermal effects, e.g. the contacts of relays, semiconductors, the windings of transformers or solenoids, can be explosion protected by enclosing them in a compound to avoid immediate contact with the environmental atmosphere. 

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