STATIC AND DIGITAL RELAYS

LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
Department of Electrical and Electronics Engineering
UNIT -V
STATIC AND DIGITAL RELAYS
Presented by,
Dr. Rohit Babu, Associate Professor

Syllabus
∟Static relays
―Static relay components
―Static over current relays
―Static distance relay
―Micro processor based digital relays

Static relay components
Static Relays
• Static relays contain electronic circuitry which may include transistors, ICs, diodes and other electronic
components.
• Static relays possess the advantages of having low burden on the CT and VT, fast operation, absence of
mechanical inertia and contact trouble, long life and less maintenance.
• Static relays have proved to be superior to electromechanical relays and they are being used for the protection
of important lines, power stations and sub-stations. Yet they have not completely replaced electromechanical
relays.
• Static relays are treated as an addition to the family of relays.
• Electromechanical relays continue to be in use because of their simplicity and low cost.
• Their maintenance can be done by less qualified personnel, whereas the maintenance and repair of static
relays requires personnel trained in solid state devices.

Static relays: These are solid state relays and employ semiconductor diodes, transistors, thyristors, logic
gates, ICs, etc. The measuring circuit is a static circuit and there are no moving parts. In some static relays, a
slave relay which is a dc polarised relay is used as the tripping device.
Definition

The component of the static relay is shown in the figure below.

Static Relay components
1. The input of the current transformer is connected to the transmission line, and their output is given to
the rectifier.
2. The rectifier was rectifying the input signal and pass it to the relaying measuring unit.
3. The rectifying measuring unit has the comparators, level detector and the logic circuit. The output
signal from relaying unit obtains only when the signal reaches the threshold value. The output of the
relaying measuring unit acts as an input to the amplifier.
4. The amplifier amplifies the signal and gives the output to the output devices.
5. The output device activates the trip coil only when the relay operates. The output is obtained from the
output devices only when the measurand has the well-defined value. The output device is activated and
gives the tripping command to the trip circuit.

6. The static relay only gives the response to the electrical signal. The other physical quantities like heat
temperature etc. is first converted into the analogue and digital electrical signal and then act as an
input for the relay.

ADVANTAGES OF STATIC RELAYS OVER ELECTROMAGNETIC RELAYS
i. Low burden on CTs and VTs. The static relays consume less power and in most of the cases they draw
power from the auxiliary dc supply
ii. Fast response
iii. Long life
iv. High resistance to shock and vibration
v. Less maintenance due to the absence of moving parts and bearings
vi. Frequent operations cause no deterioration
vii. Quick resetting and absence of overshoot
viii. Compact size
ix. Greater sensitivity as amplification can be provided easily
x. Complex relaying characteristics can easily be obtained
xi. Logic circuits can be used for complex protective schemes

1. The components used by the static relay are very sensitive to the electrostatic discharges. The
electrostatic discharges mean sudden flows of electrons between the charged objects. Thus special
maintenance is provided to the components so that it does not affect by the electrostatic
discharges.
2. The relay is easily affected by the high voltage surges. Thus, precaution should be taken for
avoiding the damages through voltage spikes.
3. The working of the relay depends on the electrical components.
4. The relay has less overloading capacity.
5. The static relay is more costly as compared to the electromagnetic relay.
6. The construction of the relay is easily affected by the surrounding interference.
LIMITATIONS OF STATIC RELAY

Static Overcurrent relay
1. Static Instantaneous Overcurrent Relay
1. The current derived from the main CT is fed to the input transformer which gives a proportional output
voltage.
2. The input transformer has an air gap in the iron core to give linearity in the current/voltage relationship
up to the highest value of current expected, and is provided with tappings on its secondary winding to
obtain different current settings.
3. 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.

4. A fixed portion of the rectified and filtered voltage (through a potential divider) is compared against a preset
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.
5. The output device may either be a static thyristor circuit or an electromagnetic slave relay.

2. Static Definite Time Overcurrent Relay
The operating time of a definite time overcurrent relay is constant, irrespective of the level of
the fault current.

1. The input current signal derived from the main CT is converted to a proportional voltage signal by
the input transformer and then rectified, filtered and compared with the preset threshold value of
the level detector (1).
2. If the voltage exceeds the preset threshold value, the level detector gives an output voltage, thereby
the charging of the capacitor C of the RC timing circuit starts.
3. As soon as the voltage across the capacitor exceeds the preset threshold value (VT) of level detector
(2), a signal through the amplifier is given to the output device which issues the trip signal.
4. Potentiometers P1 and P2 are used for current setting and time setting, respectively.

If VT is the threshold value of the level detector, the time TC required to reach this voltage depends
upon the charging time of the capacitor C of the RC timing circuit, given by,
where V is the voltage applied to the capacitor.
If V, R and C are constant, the charging time for a given value of VT will be constant.
The time TC can be varied by varying R-C combinations and VT.

3. Static Inverse-time Overcurrent Relay
The operating time of the inverse-time overcurrent relay decreases with increasing fault current.

1. The current signal is converted to a proportional voltage signal by the input transformer and
then rectified, filtered and compared with a reference voltage of the level detector (1) set by the
potentiometer P1.
2. Under normal conditions, i.e. when the input current is low, switch S1 is ON, short-circuiting the
capacitor C of the RC timing circuit and switch S2 is OFF.
3. As soon as the input voltage exceeds the preset reference voltage of the level detector (1), 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.
4. Switches S1 and S2 are made of static components.
5. When the voltage across the capacitor C of the timing circuit exceeds the reference voltage of the
level detector (2) as set by potentiometer P3, a signal is given to the output device through an
amplifier. Finally, the output device issues the trip signal.

Static Distance relay
4. Static Directional Overcurrent Relay
1. The directional overcurrent relay incorporates a directional unit which responds to power flow in
a specified direction.
2. The directional relay senses the direction of power flow by means of a phase difference (f)
between voltage (V) and current (I).
3. When f exceeds a certain predetermined value and the current is above the pick-up value, the
directional overcurrent relay operates.
4. The directional relay is a double actuating quantity relay with one input as current I from CT and
the other input as voltage V from VT.
which can be used either as a directional relay or as a distance relay depending on the input
quantities being fed.

1. The inputs V and I are applied to phase comparator.
2. A phase shifter is added in voltage input circuit before applying it to the phase comparator to
achieve the maximum output of the phase comparator under fault conditions.
3. The output of the phase comparator is given to the level detector and then to the output device
through an amplifier.

4. If the output of the phase comparator exceeds the preset reference voltage of the level detector,
the output device issues the trip signal.
5. There are two main types of phase comparators used for the purpose.
6. One of these is the Hall effect types comparator which has been used in USSR, whereas all other
countries have preferred the rectifier bridge type of comparator due to its lower cost and the
higher outputs obtainable as compared to the Hall elements.

Microprocessor Based Relay
Microprocessor relays provide many functions that were not available in electromechanical or solid-state
designs.
Relay logic is very important to understand the microprocessor-based relay.
The relay can be ON or OFF, that is, it has two stable states.

1. Similarly, the output of a logic function is activated, that is, high or deactivated, that is, low.
2. The three basic logical functions are: AND, OR and NOT functions.
3. All of these functions can be achieved using transistors as well. It is called transistor-transistor
logic.
4. The output of the CT line is given to the input receiver block where the signal is processed.
5. Signal processing includes overvoltage protector, rectifier, smoothing filters, auxiliary CT, etc.,
depending on the requirements.
6. This signal is an analog signal. The A / D converter converts this into a digital signal that is accepted
by the microprocessor.
7. The microprocessor is a block of decision making.
8. The received digital signal is compared with the reference to generate the appropriate trigger signal.

10. This is a digital signal that is converted back to analog to operate the trip coil.
11. This is achieved by the D / A converter.
12. The data logger captures the data and sends it to the microprocessor when there is a request
from the microprocessor.
13. The information can be displayed with a suitable display device when taking the signal from
the microprocessor.
14. The main advantage of this relay is that it is programmable.
15. The program can handle the online calculations and make the decision accordingly.
16. Another important advantage of the microprocessor-based relay is that a microprocessor unit
can perform the retransmission operation of several systems.

Advantages of microprocesser based relay:
Thus various advantages of microprocessor based relay are:
1. Very efficient and reliable.
2. Highly accurate.
3. Very fast in operation.
4. Programmable in nature.
5. A unit can perform retransmission of several systems.
6. economical for large systems.
7. Useful for centrally coordinated backup protection.

STATIC AND DIGITAL RELAYS

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