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. 

Types of Faults in an Alternator

An Alternator is vulnerable to many types of faults.  Each of these faults can cause major damage which can be expensive to rectify and result in loss of generation.

The common faults are

1. Stator Faults
2. Rotor Faults
   
3. Operational Faults

Stator Faults

Stator faults are those which occur on the stator of the Alternator.  These faults can be categorized into

1.  Phase-to-Phase Faults which occur between two phases

2.   Phase-to-Earth faults which occur between a phase and the ground and

3.  Inter-turn Faults which occur between the turns of a winding of the same phase.

Stator faults occur due to failure of the winding insulation.  The heat generated by these faults can cause serious damage to the laminated core of the Stator.  This may require expensive  re-insulation and rebuilding.

Rotor faults

Rotor Faults  on the Alternator when the rotor winding gets grounded or short circuited.  The rotor winding is usually ungrounded; hence the first earth fault is not always obvious.  However, if  a second earth fault occurs on the rotor, the fault becomes a virtual short-circuit through the rotor body.

Operational Faults are

Overloading 

Overloading causes the flow of high currents which causes the stator winding to heat up.

Reverse Power
This occurs due to failure of the prime mover and insufficient torque supplied to the generator.

Underexcitation
Underexcitation occurs when the excitation to the generator is cut off and the Power factor goes to the leading side.  This can lead to the failure of the diodes on the rotor and pole slipping.

Negative Phase Sequence
Negative Phase sequence occurs when the Alternator is loaded in an unbalanced manner.  That is,  the current on the three phases are not balanced.  This results in heating of the Alternator rotor.

Overvoltage 

Overvoltage occurs due to failure of the excitation control system.  If the excitation input to the alternator does not match the voltage.  It can result in the voltage rising above normal levels and the risk of the winding insulation getting damaged.

Overspeeding
Overspeeding is an extremely serious and dangerous condition.  This occurs when the speed controller regulating the speed of the prime mover fails.  When the speed of the alternator rises above the nominal speed, the centrifugal forces developed within the Alternator are so enormous that the poles of a salient pole rotor can get damaged and can come out of the rotor.  This can then hit the stator and the alternator will be severely damaged.