Static relays use electronic components like semiconductors instead of mechanical parts to detect faults and operate. They have components like rectifiers to convert AC to DC, level detectors to compare values to thresholds, and amplifiers and output devices to trigger trips. The document discusses the components, types, and applications of various static relays like overcurrent, directional, differential, distance and instantaneous relays used in power system protection.

1. Staticrelays
 Components of static relays
 Block diagram of static relays
 Types of static relays
Prepared By
Mr.K.Jawahar, M.E.,
Assistant Professor
Department of EEE
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

2. Static relays or solid state relays
• Static relay is an electrical relay in which the response (or)
action is developed by electrical/magnetic/optical or other
without mechanical motion of components.
• A static relay is defined as one in which there is no armature or
other moving element, the desired response being developed by
electronics solid state, magnetic components without
mechanical motion.
• A relay designed to get a response not from a mechanical
operation but from an electrical, electromagnetic, or optical
motion.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

3. Components of static relays
• The output of a CT or PT of a transducer is rectified by the
rectifier.
• The rectified output is given to a measuring unit constitute of
comparators, level detectors, and logic circuits. The output is
actuated when the dynamic input, i.e. the relaying quantity attains
the threshold value.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

4. • The output of the measuring unit is fed to the output unit devices
after it is amplified by the amplifiers. The output unit activates
the trip coil only when the relay operates. The relaying quantity
such as the voltage and current is rectified and measured.
Working of a static relay
• The output of CTs/PTs/Transducers is rectified in rectifier.
• The rectified output is fed into the relay measuring unit.
• The output of measuring unit is then amplified in amplifier
• The amplified output is given to the output device, which
energizes the trip-coil, when the relay operates.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

5. Merits of static relays
• Reliability
• Sensitivity
• Speed
• Selectivity
• Versality
Limitations of static relays
• Auxiliary voltage requirement for relay operation
• Static relays are sensitive to voltage transients which are caused
by operation of breaker and isolator in the primary circuit of CTs
and PTs.
• Non availability of test data
• Highly reliable power supply circuits are required
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

6. Applications
• Used in Ultra-high speed protection schemes of EHV-AC
lines utilizing distance protection.
• Over current schemes
• Earth fault protection schemes
Semiconductor devices used in static relay
• Semiconductor diode
• Transistors
• Unijunction transistor
• Thyristors
• Logic circuits
• Filter circuits
• Multivibators
• Time delay circuits
• Level detectors
• Analog circuits
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

7. Static over current relays
• The output of this summation C.T is fed to an auxiliary C.T.
whose output is rectified, smoothened and applied to the
measuring unit called level detector.
(a) When the input current to the level detector is less than the
threshold value or set value, the output of the level detector is
zero.
For the over current relay,
If Iinput < Ithershold, Iout =0
If Iinput ≥ Ithershold, Iout= positive quantity
(b) The output of the level detector is amplified and applied to the
output device to cause trip/alarm.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

8. Static over current relays
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

9. Static instantaneous over current relay
• The current derived from the main C.T. is fed to the input
transformer which gives a proportional output voltage.
• The input transformer has an air gap in the iron core to give
linearity in the current/voltage relationship up to highest value
of current expected, and is provided with tappings on its
secondary winding to obtain different current settings.
• The output voltage of the transformer is rectified through a
rectifier and then filtered at a single stage to avoid undesirable
time delay in filtering so as to ensure high speed of operation.
• A limiter made of a zener diode is also incorporated in the
circuit to limit the rectified voltage to safe values even when
the input current is very high under fault conditions.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

10. • A fixed portion of the rectified and filtered voltage through a
potential divider is compared against a pre-set pick up value by
a level detector and if it exceeds the pick up value, a signal
through an amplifier is given to the output device which issues
the trip signal.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

11. Definitetime over current relay
• The operating time of over current relay is constant, irrespective of
the level of the fault current.
• The input current signal derived from the main C.T. is converted
to a proportional voltage signal by the input transformer and then
rectified, filtered and compared with the pre-set threshold value of
the level detector.
• If the voltage exceeds the pre-set threshold value the level detector
gives an output voltage, there by the charging of the capacitor ‘C’
of the RC timing circuit starts.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

12. • As soon as the voltage across the capacitor exceeds the pre-set
threshold value (VT) of level detector ‘2’ a signal through the
amplifier is given to output device which issues the trip signal.
• Potentiometers P1 and P2 is used for current setting and time
setting respectively.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

13. Inverse-Time over current relay
• Under normal conditions, when the input current is low, switch S1
is ON, short circuiting the capacitor C of the RC tunning circuit
and switch S2 is OFF.
• As soon as the input voltage exceeds the pre-set reference voltage
of the level detector A.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

14. • Switch S1 is switched off and switch S2 is switched ON and the
charging of capacitor C of the timing circuit starts from a
voltage proportional to the current.
• Switch S1 and S2 are made of static components. When the
voltage across the capacitor C of the timing circuit exceeds the
reference voltage of the level detector ‘B’ as set by
potentiometer Pz.
• Finally the output device issues the trip signal. Here the plus
setting multiplier is given by the transformer secondary tap
and potentiometer Px and the time multiplier setting is
determined by potentiometer Py and Pz.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

15. Directional Static over current relay
• The directional relay is nothing but a differential power relay
which operates when the power in the circuit flows in a
particular direction. Thus it requires to sense the system voltage
as well as the system current.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

16. • The input A is proportional to the system current supplied to a
directional unit through auxiliary transformer.
• The input B is proportional to the system voltage, supplied to a
directional unit through phase shifter.
• The phase comparator compares the phase angle between the
two points.
• The phase comparator is generally two types
(i) Hall effect generator which is popularly used in Russian
countries.
(ii) Rectifier bridge type comparator
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

17. Static differential relay
• In this relay the two similar input quantities (I or V) are compared.
The comparator is usually a rectifier bridge rectifier. The
differential relay measures the vector difference between two
similar electrical quantities say V or I.
• Static differential relay is mostly applied in protection of
generation and transformers against any type of internal fault
similar to that of electromagnetic type differential relay.
• It is very compact, highly sensitive, high stability, low power
consumption lower VA burden.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

18. Static distance relay
• In the distance relay, the operation is dependent on the ratio of
the voltage and current, which is expressed in terms of
impedance. The relay operates when the ratio V/I i.e.,
impedance is less than a predetermined value.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

19. • The measurement of impedance, reactance, and admittance are
done by comparison of input current and voltage. In static
comparators the two input quantities are either V or I.
• Current is converted to equivalent voltage by producing a
voltage drop in impedance with in the relay.
• This voltage drop is then compared with the other voltage.
• The output from the comparator is fed into a polarity detector
where, when the input currents to the comparator are 90 apart,
the output device will be turned on for +90 and turned -90 and
hence the output wave will be a square wave with equal space
ratios.
• Static distance relay are used extremely for protection of
medium and long transmission lines, parallel feeders and unit
back up protections as well as interconnected lines.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

20. Reference:
• V.Thiyagarajan, “ Protection and Switchgear”, Lakshmi
Publications,2018.
• B.Rabindranath and N.Chander, ‘Power System Protection and
Switchgear’, New Age International (P) Ltd., First Edition
2011.
Static Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE