Question

Which one of the following types of static relay uses a polarity detector as a component?
1. Static overcurrent relay
2. Static directional relay
3. Static differential relay
4. Static distance relay

Answer 1

Correct Answer - Option 4 : Static distance relay

Static Relay:

It is an electrical relay, whose output response comes from an electric or magnetic component without mechanical motion.

Merits of static relay:

Reliability, sensitivity, speed etc.

Applications:

• Used in ultra-high speed protection scheme.
• Earth fault protection
• Over-current protection

 

Static distance relay:

• Ideal static distance relays have characteristics independent of actual magnitudes of voltage and current but dependent only on their ratio and phase angle between them.
• A polarity detector is an important characteristic in current measurement circuits of protection secondary systems as it can damage the equipment.
• The measurement of reactance, impedance and admittance are done by comparing the input voltage and current.
• Used for the protection of medium and long transmission lines, parallel feeder.

 

Few static relays:

1. Static overcurrent relay:

• The output of summation CT is given to an auxiliary CT.
• Output is rectified, smoothened and supplied to the measuring unit (level detector).
• The measuring unit determines whether the quantity has attained the threshold value (set value) or not.
• After operation of the measuring unit, the output is amplified by the amplifier. The amplified output is given to the output circuit to cause trip/alarm.

 

2. Static directional relay or Directional overcurrent static relay:

• The static directional relay with two inputs (V and I). The inputs are supplied to the phase comparator.
• A phase shifter is included in the voltage input circuit, whose output is fed to the phase comparator.
• The output of phase comparator is fed to the level detector and after that, it is amplified and in case a timer is necessary, the output is applied to the output device through the timer.

 

3. Static differential relay:

• Measures the phasor difference between two similar electrical quantities (voltage-voltage or current-current).
• The output of the comparator is amplified and used to operate the relay.
• Use for the protection of transformer and generator from internal faults.