Q2Protection of Power System

A thermal relay (thermal overload relay) protects a piece of equipment, most commonly an induction motor, against sustained overload conditions by monitoring the heating effect of the load current rather than the current magnitude directly, closely mimicking the actual thermal heating behavior of the protected motor winding itself.

Construction

The most common construction uses one or more bimetallic strips, each formed from two dissimilar metals with different coefficients of thermal expansion bonded together along their length, arranged so that current flowing either directly through the strip itself (direct heating) or through a separate heater element wound closely around the strip (indirect heating, more common in practice since it allows the heater element's resistance to be independently selected to match the specific current rating and heating characteristic required, decoupled from the mechanical properties needed for the bimetallic strip material itself) causes the strip to heat up in proportion to the load current (and, since heating is a resistive I-squared-R effect, in proportion to the square of the current).

Principle of Operation

As the bimetallic strip heats up, the differential thermal expansion of its two bonded metal layers causes the strip to bend or curl in a direction determined by which metal layer has the higher expansion coefficient; this bending motion is mechanically coupled to a set of relay contacts (or, in a motor starter application, directly to the trip mechanism of the associated contactor), such that sufficient bending (corresponding to a sufficiently high winding temperature, itself the result of a sustained overload current) causes the contacts to operate, tripping the protected circuit. Because the bimetallic strip's temperature rise (and hence its bending) depends on the thermal time constant of the strip itself, which is deliberately engineered to approximate the thermal time constant of the actual motor winding being protected, the thermal relay closely emulates the true thermal state of the motor, allowing it to ride through brief, high-current transients (such as motor starting inrush current) that do not represent a genuine sustained overload danger, while still reliably tripping for genuinely sustained overload conditions that would otherwise cause the motor winding to overheat and its insulation to degrade.

Working with Schematic Considerations

In a typical three-phase motor thermal overload relay, three separate bimetallic strips (one per phase) are each heated by a heater element carrying the corresponding phase's load current, with a common mechanical linkage (a tripping bar or similar mechanism) arranged so that any single phase's bimetallic strip bending sufficiently will operate the common trip contacts, providing protection not only against a balanced three-phase overload but also against a single-phasing condition (loss of one supply phase), since the remaining two phases would then carry increased current, causing their corresponding bimetallic strips to heat and bend more rapidly than under balanced three-phase loading, tripping the relay before the motor winding is damaged by the resulting unbalanced heating.

Directional Relay

A directional relay is a protective relay that operates only when both the magnitude of the actuating quantity (typically current) exceeds a set threshold and the direction of power (or current) flow relative to a reference direction matches the relay's designed tripping direction, remaining restrained (non-operating) for fault currents flowing in the opposite direction even if their magnitude alone would otherwise be sufficient to cause an ordinary, non-directional overcurrent relay to operate. Directional relays are essential in protecting parallel feeders, ring main distribution systems, and interconnected networks where fault current can flow toward a given relay location from either direction depending on the location of the actual fault, since a purely non-directional overcurrent relay cannot distinguish between a fault requiring that relay to trip and a fault elsewhere in the network that happens to also cause current of sufficient magnitude to flow through that relay's location but in the reverse (non-fault) direction, where tripping would be incorrect and would unnecessarily disconnect a healthy section of the network. A directional relay achieves this direction-sensing capability by comparing the phase relationship between the actuating current and a reference (polarizing) quantity, typically a voltage derived from the system at the relay location, since the phase angle between fault current and this reference voltage differs characteristically depending on whether the fault lies in the relay's intended tripping direction or in the reverse direction.

90 Degree Connection Method

The 90 degree connection is a standard, widely used method of connecting the polarizing voltage to the current coil of a directional relay for each phase, chosen because it gives maximum torque (maximum sensitivity) for typical power system fault power factors, which for most faults (particularly those involving a significant inductive reactance component) tend to lag the corresponding unfaulted-phase voltage by an angle in the range of roughly 60 to 90 degrees. In the 90 degree connection, the current coil of the directional element for a given phase (for example, phase A) is polarized using the line-to-line voltage between the other two phases (phase B and phase C), specifically the voltage Vbc, rather than using the phase A voltage itself; this specific choice of polarizing voltage is called the 90 degree connection because, under balanced system conditions and unity power factor load current, the phase A current and the Vbc polarizing voltage are in quadrature (90 degrees apart), and this quadrature relationship shifts the relay's maximum torque angle to align well with the phase angle characteristically exhibited by actual inductive fault currents relative to the healthy-phase voltages, giving the directional element its best sensitivity and most reliable directional discrimination specifically for the predominantly lagging power factor fault currents typically encountered in practical power systems, in contrast to alternative connection schemes (such as the 30 degree or 60 degree connections) that shift the maximum torque angle to better suit different assumed fault power factor characteristics.

It is further worth noting that the 90 degree connection discussed here is one of several standard polarizing connection schemes (others include the 30 degree and 60 degree connections), each named for the phase angle relationship between the polarizing voltage and the relevant phase current at unity power factor, and the choice among these standard connections in a specific protection scheme design is generally made to best match the expected power factor characteristic of faults anticipated on the specific type of protected element (transmission line, generator, or transformer feeder) at that particular location in the system, since different types of faults and different system voltage levels can exhibit somewhat different characteristic fault power factors.