Motor protection relays--part 2 [Industrial Electrical Power Systems]

Steady-state temperature rise

In the interest of maximum efficiency, electrical machines should be loaded as close as possible to their permitted operating temperature limit; however, excessive thermal stressing of any appreciable duration must be avoided if the life of the insulation is not to be shortened.

Under steady-state conditions, the temperature of a motor will rise exponentially, due to dissipation of the heat to the environment or cooling medium, towards its respective operative temperature. Since a motor is not a homogeneous mass, heat is dissipated in several stages. Temperature rise and fall takes place according to a series of partial time constants.

In spite of this, it’s sufficient for a thermal overload relay intended for protection under steady-state conditions to be set to the mean time constant of the motor. This means that proper account is taken only of the copper losses. Measurement of the voltage would be necessary in order to include the iron losses, but is not generally possible since the voltage transformers are usually located on the busbar and not adjacent to each motor.

Most modern thermal overload relays only measure current, filtering out the highest of the three-phase current. The critical cases of starting, stalling and failure of a phase are taken care by other protective functions.

100% 63% T t

N = rated temperature rise IN = rated current t = duration of temperature rise T = time constant

++++ Temperature rise vs time (illustrating time constant)

Thermal time constant

The time constant T (tau) is defined (IEC 255-8) as the time in minutes required for the temperature of a body to change from an initial temperature ?0° to 63% of the difference between ?0° and the new steady-state temperature ??. Unfortunately, the thermal time constant T of the motor is frequently not known. +--+ 17.1 gives typical values in relation to motor ratings and mechanical design. The cooling time constants during operation are approximately equal to those for temperature rises, while at standstill they are 4-6 times the values given in the table.

Type A[mm]

1250 O 20 25 28 30 35 40 50 60 65, 70 R 45 50 55 60 70 80 90 100 110 U 30 35 40 45 50

= Shaft height (mm)

= Open type (IP23)

= Closed type with air/air heat-exchanger (IP54)

= Fully clad with cooling finds (IP54)

+--+ Mean thermal time constants of asynchronous motors from Brown Boveri in relation to motor rating and type

Motor current during start and stall conditions

As the magnitude and duration of motor starting currents and the magnitude and permissible duration of motor stalling currents are major factors to be considered in the application of overload protection, these will be discussed. It’s commonly assumed that the machines started direct-on-line the magnitude of the starting current decreases linearly as the speed of the machine increases. This is not true. For normal designs, the starting current remains approximately constant at the initial value for 80-90% of the total starting time. When determining the current and time settings of the overload protection, it can be assumed that the motor starting current remains constant and equal to the standstill value for the whole of the starting time.

++++ Motor current during start conditions

Motor stall

Should a motor stall when running or be unable to start (run) because of excessive load, it will draw a current from the supply equivalent to its locked rotor current. It’s obviously necessary to avoid damage by disconnecting the machine as quickly as possible if this condition arises. It’s not possible to distinguish this condition from a healthy starting condition on current magnitude.

Current Time TR TS Stall current-- Relay thermal characteristic-- Direct-on-line motor starting characteristic TR Motor thermal withstand characteristic-- Rotor protected TR < TS IS --Stall withstand time

++++ Relay operation time less than stall withstand time: relay gives stall protection

Current Time TR TS Stall current Relay thermal characteristic Direct-on-line motor starting characteristic TR Motor thermal withstand characteristic Rotor not protected TR < TS IS Stall withstand time

Relay operation time greater than stall with stand time: relay does not give stall protection The majority of loads are such that the starting time of normal induction motors is about or less than 10 s, while the allowable stall time to avoid damage to the motor insulation is in excess of 15 s.

If a double cage drive is to be protected, it might be that the motor cannot be allowed to be in a stall condition even for its normal start-up time. In this case, a speed switch on the motor shaft can be used to give information about whether the motor is beginning to run up or not. This information can be fed to suitable relays, which can accelerate their operating time. Whether or not additional features are required for the stalling protection, depends mainly on the ratio of the normal starting time to the allowable stall time and the accuracy with which the relay can be set to match the stalling time/current curve and still allow a normal start.

Unbalanced supply voltages

The voltage supplied to a three-phase motor can be unbalanced for a variety of reasons; single-phase loads, blown fuses in pf capacitors, etc. In addition, the accidental opening of one-phase lead in the supply to the motor can leave the motor running, supplied by two phases only.

It might seem that the degree of voltage unbalance met within a normal installation (except when one-phase is open circuited) would not affect the motor to any great extent, but this is not so. It should be remembered that it’s not the unbalanced voltage which is important, but the relatively much larger negative sequence component of the unbalance current, resulting from the unbalanced voltage.

Closed, Open Speed-switch; Motor current; Motor speed; Current sig.; Relay; Speed switch; Blocked rotor signal

The following are typical of a normal motor start:

++++ Typical motor start; Closed Open; Closed, Open: Motor current, Motor speed, Current sig. relay, Speed switch Blocked rotor signal; The following characterize a blocked rotor condition:

++++ Blocked rotor condition

The method of symmetrical components consists of reducing any unbalanced three phase systems of vectors into three balanced systems: the positive, negative and zero sequence components. The positive sequence components consist of three vectors equal in magnitude 120° out of phase, with the same phase sequence or rotation as that of the source of supply. The negative sequence components are three vectors equal in magnitude, displayed by 120° with a phase sequence opposite to the positive sequence. The zero sequence components consist of three vectors equal in magnitude and in a phase.

Rotation:

++++ The positive, negative and zero components Loss of one-phase represents the most dangerous case of unbalance. It’s therefore essential for motors, which are protected against short circuit by fuses (limited breaking capacitor of the breaker) to be equipped with fast-operating loss of phase protection.

Determining sequence currents

In the general case of unbalanced three-phase voltages, there is no fixed relationship between the positive and negative sequence currents; the actual value of the negative sequence current depends on the degree of unbalanced supply voltage, and on the ratio of the negative to the positive sequence impedance of the machine. The ratio can be determined from the full-load-rated torque for a voltage unbalance in the order of 10% and can therefore be neglected. Hence, presence of negative sequence currents does not appreciably affect the starting characteristics.

The main effect of the negative sequence current is to increase the motor losses, mainly copper loss, thus reducing the available output of the machine if overheating of the machine windings is to be avoided. The reduction in output for the machines having ratios of starting to running current of 4, 6 and 8 respectively is shown for various ratios of negative to positive sequence voltage.

Maximum continuous output (%)

Voltage unbalance × 100% E2

++++ Maximum continuous output vs voltage unbalance 17.10 Electrical faults in stator windings ground faults phase-phase faults

Ground faults

Faults, which occur within the motor windings are mainly ground faults caused by breakdown in the winding insulation. This type of fault can be very easily detected by means of an instantaneous relay, usually with a setting of approximately 20% of the motor full-load current, connected in the residual circuit of three current transformers.

Care must be taken to ensure that the relay does not operate from spill current due to the saturation of one or more current transformers during the initial peak of the starting current; this can be as high as 2.5 times the steady-state rms value, and may cause operation, given the fast-operating speed of the normal relay. To achieve stability under these conditions, it’s usual to increase the minimum operating voltage of the relay by inserting a stabilizing resistor in series with it. Thermal overload

Locked rotor

Phase fault

(optional) Ground fault

Ground fault (alternate connection)

Current unbalance

Ammeter display 50N 51N

Preferred R_stab

F1 not covered (more sensitive) Armoring; Ground screen; Core balance

++++ Ground fault protection

Phase-phase faults

Because of the relatively greater amount of insulation between phase windings, faults between phases seldom occur. As the stator windings are completely enclosed in grounded metal, the fault would very quickly involve ground, which would then operate the instantaneous ground fault protection described above.

Differential protection is sometimes provided on large (2 MW) and important motors to protect against phase-phase faults, but if the motor is connected to an grounded system there does not seem to be any great benefit to be gained if a fast-operating and sensitive ground fault is already provided.

Terminal faults

High-set instantaneous overcurrent relays are often provided to protect against phase faults occurring at the motor terminals, such as terminal flashovers. Care must be taken when setting these units to ensure that they don’t operate on the initial peak of the motor starting current, which can be 2.5 times the steady-state rms value.

The asymmetry in the starting current rapidly decreases, and has generally fallen to its steady-state value after one cycle. A typical motor starting current.

Percent full load current

Time (ms): 10ms = 0.1% Ns

++++ Transient overcurrent during first few cycles when starting a motor.