Variables used for Electrical Fault Detection
Over Current is defined in a power system when the maximum continuous load current permissible for an item of electrical plant is exceeded. Over currents arise due to phase and ground faults in the power system and during normal operation (motor Starting, energizing the power transformers..) .
Over Current is used as important variable to detect the phase and ground faults.This variable (Over Current) is used for many reasons such as Simplicity and less cost, they are more frequently applied in the overload protection.
Over Current protection devices continuously monitor the current being conducted by the protected unit and issue a tripping command to the circuit breaker when the current exceeds the current setting to isolate the respective part of the power system from the other.
The pickup of the Over Current Protection in the case of an induction motor is normally set higher than the maximum AC starting current (Starting current of the Induction is motor is generally 5 times the full load current)
In Differential or Circulating current protection the amplitude of current upstream and downstream of the protected unit are compared. Faults is detected when the difference between the two currents is no longer zero. This type of protection is used to identify the fault inside the particular zone. This sort of protection is employed to identify the internal faults in the Transformer and Alternators.
The figure shows the differential current protection for a three phase transformer. 2 Current transformers are connected to the unit for providing differential protection.
During normal Operation, i1and i2 are equal so no current flows through the OC coil. When external fault occurs still i1 and i2 are same and the relay do not respond to the fault external to the protection unit. When the Internal Fault occurs the current i1 and i2 are not equal so a circulating current flows through the loops and current flows through the OC coil which energizes the relay thereby opening the circuit breaker.
Difference of Current Phase angles:
This type protection compares the phase angles of the current flowing in and out of the protected circuit. This is therefore referred to as phase comparison protection. In differential or Circulating protection the amplitude of both the sides of the protection unit is measured, in this case the difference between the phase angles is measured.
Over and Under Voltage
Over Voltages in a power system is the result of defective voltage regulators on generators or power transformers, or because of load shedding, or because of poor power factor regulation. Atmospheric Over voltages are because of the lightning strikes and travelling waves, but they will be taken care by Lightning Arresters. Under Voltage is because different faults in the power system.
Generally sometimes, the voltage criteria is combined with the another criteria is employed depends on the application.
A Directional unit is used in combination with an Overcurrent unit in the power system where overcurrent criteria is not sufficient to discriminate. The principle of power direction is also used along with the over current principle in certain applications.
Directional protection is also employed in the large turbo generators to detect the abnormal operation of the alternator as motor
The deviation of the frequency of an power system from its rated value ia an indication of an imbalance between real power generation and load demand . If the demand for real power is more than power generated, then the frequency falls down on the grid on the other hand when surplus amount of real power is available and very less load exist, then frequency tend to increase.
Inorder to have a synchronous operation in the grid the frequency of the grid should be in a certain band. Frequency deviations are relatively small and the fluctuations will be in the order of mHz/MW range for a change in the real power. Thus frequency is an important parameter to understand the faults in the grid and helps in maintaining the stability of the grid. When the load demand is more than generation then, frequency tend to fall down. This process depends on the amount of the deficit occurred in the real power. Inorder to restore to the balance situation load connected to grid is removed in steps and this process is called load shedding.
Measurement based on Frequency (f) and Rate of Change of Frequency (df/dt) is an important parameter to balance the real power in the grid.Frequency relays are used in the power system to maintain the synchronism in the grid.
Types Of Faults
Variables Used For Detection
Phase Current, Current Difference, Phase Angle, Phase Voltage, Power Direction, Impedence
All the above variables can be used and also the Negative and Zero Sequence Components of Currents, Voltages and Power Directions
Zero Sequence Components of Currents, Voltages, and Power Direction
Phase Current and Temperature
Real Power Deficit
Frequency, Rate Of Change Of Frequency
Real Power Excess