Page 230 - Offshore Electrical Engineering Manual

Conventional Relay Types 217

overheating could be tolerated, for example, an Ex ‘e’ motor in a hazardous area, may
also require such a device to be fitted. In ATEX-certified machines, the overload is
certified as part of the motor certification.

STALLING PROTECTION

When the elevated current drawn by a motor during starting is prolonged by, for
example, a mechanical fault in the motor or driven machinery, then the motor will
very rapidly overheat, unless quickly disconnected from the supply. If the probabil-
ity of a motor stall is considered significant for a particular drive package, a stalling
relay should be fitted. This device is a thermal element similar to the overload ele-
ment but designed to respond to the higher short-term starting currents and to trip
if the starting current continues for more than 5–10 s beyond the normal starting
time. Normal starting times vary depending on the characteristic of the driven equip-
ment, and as they increase, discrimination between starting and stalling conditions
becomes more and more difficult, until the point where it becomes necessary to fit a
tachometric device to the drive shaft in order to detect the absence of rotation or the
failure to achieve normal running speed.

PHASE UNBALANCE PROTECTION
The loss of one phase of the supply or unbalanced supply voltages will have two
effects. First, it will create a negative phase sequence current which will cause addi-
tional rotor heating. Second, it will cause excessive heat in the stator windings if
the current in one or more phases exceeds the normal full-load value. The heating
effect due to the negative sequence current can pose a greater threat than that due to
the unbalanced phase currents, and therefore it is advisable, especially with larger
machines, to provide a relay with negative phase sequence detection. The operation of
the phase imbalance facility on the P&B Golds relay is described in the next section.

CONVENTIONAL RELAY TYPES

The most common of the conventional thermal overcurrent relay offshore is the P&B
Golds relay. The relay consists of three tapped saturating core current transformers
(CTs) which are fed from the main protection CTs. The secondary of each saturating
core CT feeds a helical heating coil element which operates its own moving contact.
The construction of the basic thermal overload element is shown in Fig. 4.3.1. The
centre phase heater is used to provide ‘% Running Load’ indication, overload trip
when the moving contact touches the ‘% Load to Trip’ contact and unbalanced load
trip when contact with either of the outer phase contacts is made. Under healthy
operating conditions, the out-of-balance contact cradle is horizontal so that no con-
tacts are touching. However, if the motor load becomes unbalanced or single phasing
occurs, the differential heating effect will cause the cradle to tilt over until the con-
tacts touch and the motor contactor is tripped.