Protection Relays - Part 3

Grading of Overcurrent Relays

Grading of over-current is the adjustment of the settings of the over-current relays to ensure discrimination and selectivity. Consider a radial feeder with multiple feeders in series. An over-current protection relay is installed at every breaker location. When a fault occurs at any given point, only the relay located closest to the fault should operate.  This is known as grading.

There are different types of grading. They are
1)   Current Grading
2)   Time Grading
3)   Time-Current Grading

Current Grading

Current Grading refers to the discrimination achieved by reducing the current setting as we move towards the power source. This ensures that the relay closest to the fault trips first. The downside of this arrangement is that the fault current does not always vary with the location. Hence, it is not possible to accurately discriminate between the relays.

 Time Grading

Time Grading refers to the discrimination achieved by varying the time delay for the different relays. In this method, the relay farthest from the source has the shortest time delay and the time delay increases as we move towards the source. That is, the source breaker will have the highest time delay. This will work in systems where the fault current is uniform across the system. However, this type of grading will not be sufficient in systems where the fault current varies with the location of the fault.

Time-Current Grading System

The Time current grading system is the most widely used method of Grading. This method uses a combination of Time and Current grading to achieve discrimination. In this method, the time setting varies with the fault current. A severe fault will have a shorter time delay  while the delay will be more for a mild fault.

Selection of Current Setting

The current setting is determined by first calculating the current during a fault. This is done by a procedure called the fault level calculation. The current during a fault will depend on the number of power of upstream power sources. Thus the fault current at minimum generation and the fault current at maximum generation should be calculated. A three phase fault during maximum generation will cause maximum fault current while a fault between two phases during minimum generation will result in minimum fault current.

Each section of the distribution should serve as a backup for the immediate section downstream. The setting such that the relay operates for a fault at the adjoining section during minimum generation. The current setting is lowest at the feeder farther from the source and increases towards the source.

Voltage Supervision Relays

The Voltage Supervision Relay is an integral part of any protection system. The voltage supervision relay protections systems from undervoltage and overvoltage. Overvoltage in a system can result in serious damage to insulation and equipment while undervoltage can cause motors to draw more current and reduce the speed of the motors, disturbing the process.

Besides protecting against overvoltage, the voltage supervision relay can also be used to detect earth faults as the phase to earth voltage is distorted when there is an earth fault in one of the phases. Voltage supervision relays can generate alarms when the voltage is low or high in only one phase. This is also known as phase asymmetry.

In motor circuits, the voltage supervision relay protects against single phasing. Single phasing can cause serious damage to motors.

A simple auxiliary relay can also be used to generate alarm for undervoltage. When the voltage drops, the relay can drop off thus generating an alarm or a shutdown.

Rotor Earth Fault Relay

The Rotor Protection relay is used in synchronous motors and generators to identify the presence of an earth fault in the rotor winding. While the winding in the rotor is insulated from the ground during normal operation, the Rotor is subjected to stresses due to vibration, heat, etc. These stresses can cause the winding to give way in a particular place and the winding can get earthed.

While a single earthing in the winding is not immediately damaging. It sets the stage for damage if a second failure should occur. The second earthing can cause a short-circuit through the rotor causing extensive damage to the rotor and the winding.

The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic balance inside the alternator. These forces can cause the rotor shaft to become eccentric and in extreme cases cause bearing failure.

Hence, it is necessary that any earthing in the rotor is detected at the earliest.

In slip ring rotors, carbon deposits on the slip rings may compromise the insulation resistance of the rotor. Hence, the slip rings need to be inspected for any deposits.

The Rotor Earth Fault Protection Device consists of a current injection device which applies an AC voltage to the rotor winding by means of a slip ring fitted on the rotor. The current is applied to the rotor through a coupling capacitor. In the normal condition, the system is floating and the current flowing through the device is zero as the resistance is high.

When a fault occurs, the current increases causing the relay to operate. The relay can be configured for alarm or trip depending on the criticality.