RELAYS, A PROTECTION SYSTEM

WHAT IS A RELAY?

 Device which receives a signal from the power system and determines whether conditions are
"normal" or "abnormal" (measuring function)-
 If an abnormal condition is present, relay signals circuit breaker to disconnect equipment that could be
damaged (Switching or signaling function)
 "Relays" send signal from system to circuit breaker.
 What is the Purpose of the Relay?
• The purpose of the protective relaying systems is to isolate only the faulty
component of power system.
• Relaying equipments are classified into two groups:
1. Primary relaying equipment.
2. Back-up relaying equipment.
• Primary relaying is - the first line of defense for protecting the equipments.
• Back-up protection relaying works only when the primary relaying
equipment fails (they are slow in action).
 Desirable Relay
Characteristics
■ Speed (1/60 sec)
* Minimizes damage from current
* Maximizes power transfer during normal conditions, stability

■ Security
* Relay should not cause circuit breaker to open during normal conditions

■ Dependability
* Relay should cause circuit breaker to open during abnormal conditions

■ Sensitivity
 Ability of a relay to detect al faults for the expected limiting system and fault conditions

■ Selectivity
Ability of a relay system to discriminate between faults internal and external to its intended
protective zones.
CLASSIFICATION OF RELAYS
Protection relays can be classified in accordance with the function which they carry out,
their construction, the incoming signal and the type of protection.

General function:
• Auxiliary. Protection. Monitoring. Control.

Construction:
 Electromagnetic.
 Solid state.
 Microprocessor.
 Computerized.
 Nonelectric (thermal, pressure ......etc.).
CLASSIFICATION OF RELAYS…

Incoming signal:
 Current.
 Voltage.
 Frequency.
 Temperature.
 Pressure.
 Velocity.
 Others.
Type of protection
 Over current.
 Directional over current.
 Distance.
 Over voltage.
 Differential.
 Reverse power.
 Other.
DEFINITIONS
■Normally open contact ( N/O):
■is one which is open when the relay is not energized.
■Normally closed contact (N/C):
■ is one which is closed when the relay is not energized.

■Operating force or torque:

■that which tends to close the contacts of the relay.

■Restrain force or torque:

■ that which opposes the operating force or torque and tend to prevent the closure of the relay contacts.

■ Pick-up level:
■ the value of the actuating quantity (current or voltage), which is on the border above which the relay
operates.

■ Drop-out or reset level:

■ the value of current or voltage below which a relay opens its contacts and comes to original position..
DEFINITIONS..

■ Operating time:
■ the time which elapses between the instant when the actuating quantity exceeds the pick-up value to
the instant when the relay contacts close.
■ Reset time:
■ the time which elapses between the instant when the actuating quantity becomes less than the reset
value to the instant when the relay contact returns to its normal position.
■ Primary relays:
■ the relays which are connected directly in the circuit to be protected.
■ Secondary relays:
■ the relays which are connected in the circuit to be protected through CTs and V.Ts.
■ Auxiliary relays:
■ relays which operate in response to the opening or closing of its operating circuit to assist another
relay in the performance of its function. This relay may be instantaneous or may have a time delay.
■ Reach:
■ a distance relay operates whenever the impedance seen by the relay is less than a prescribed value,
this impedance or rt]ic corresponding distance is known as the reach of the relay.
■ Instantaneous relay:
■ One which has no intentional time-delay and operates in less than 0.1 second.
■ Blocking:
■ preventing the protective relay from tripping cither due to its own characteristics or to an additional
relay.
■Time delay relay : One which is designed with a delaying means .
 Types of Relay

Solid Electromagnetic Computerized

State
Others:
Temperature
Pressure

Magnetic Magnetic Digital Numerical

Induction Attraction Relays Relays

Attracted Plunger
Armature Type

Wattmetric Shaded Pole

Induction
Type
Cup
ELECTROMAGNETIC RELAYS
• Research Began at the End of the 19th Century
• The Relay Family Was Completed in the 1930’s
• They Are Still in Use
• These relays were the earliest forms of relay used for the protection of power systems, and
they date back nearly 100 years.
• They work on the principle of a mechanical force causing operation of a relay contact in
response to a stimulus.
• The mechanical force is generated through current flow in one or more windings on a
magnetic core or cores, hence the term electromechanical relay.
• The principle advantage of such relays is that they provide galvanic isolation between the
inputs and outputs in a simple, cheap and reliable form –
• Therefore for simple on/off switching functions where the output contacts have to carry
substantial currents, they are still used.
ELECTROMAGNETIC RELAYS…

They can be classified into several different types as follows:

• magnetic attracted armature relays
• magnetic induction relays
• moving coil
• thermal
• However, only attracted armature and induction types have significant application
at this time, all other types having been superseded by more modern equivalents.
• Electromagnetic relays are constructed with electrical, magnetic and mechanical
components, have an operating coil and various contacts and are very robust,
inexpensive and reliable.
• However they required maintenance by skilled personnel.
Magnetic attraction relays
Magnetic attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal when it is
attracted by the magnetic field produced by a coil.
There are two main types of relay in this class.
 Attracted armature type (clapper type)
 Plunger type
The attracted armature relays: which are shown in Fig., consists of a bar or plate of metal which pivots when
it is attracted towards the coil.

(a) (b)

Fig. Attracted armature-type relays

 ATTRACTED ARMATURE TYPE

 The armature is attracted to the electromagnet when the current reaches a certain
predetermined value (iop – operating current).
 The force of the armature will trip the link mechanism of the circuit breaker, or it may
operate as a relay and close the contacts of a separate tripping circuit.
 The armature is attracted against gravity or a spring.
 By adjusting the distance of the armature from the electromagnet, or the tension of the
spring, the current at which the trip operates can be varied to suit the circuit conditions.
 The armature carries the moving part of the contact, which is closed or opened according
to the design when the armature is attracted to the coil.
 PLUNGER TYPE

• Plunger type relay: The other type is the piston or solenoid relay, illustrated in
Figure 2, in which α bar or piston is attracted axially within the field of the
solenoid. In this case, the piston also carries the operating contacts. This called
plunger type relay.
 It can be shown that the force of attraction is equal to K1I2 - K2, where Κ1
depends upon the number of turns on the operating solenoid, the air gap, the
effective area and the reluctance of the magnetic circuit, among other factors.
K2 is the restraining force, usually produced by a spring.
 When the relay is balanced, the resultant force is zero and therefore Κ112 = K2,
so that :
Solenoid Type or plunger
• I  K 2 / K1 constant.
type
 Induction Type Relay with plug settings

Fig. Induction type relay with Plug settings

Induction-Cup relay
• The operation is similar to the induction disc; here, two fluxes at right
angles induce eddy currents in a bell-shaped cup which rotates and carries
the moving contacts.
• A four-pole relay is shown in Figure.

Fig .Four-pole induction-cup relay.

 Shaded-pole relay
• In this case operation of the electromagnetic section is short-circuited by means of a copper ring or coil.
• This creates a flux in the area influenced by the short circuited section (the so-called shaded section) which
lags the flux in the nonshaded section

Fig. Shaded – pole Relay

Note that the main coils has TAPS, this means that the number of turns is actually adjustable.

• In the electromagnetic induction principle, the relay element has a non-magnetic rotor (an aluminum or copper disc or
cylinder) in which coils create magnetic fluxes that induce circulating currents.
• The interaction between the fluxes and the circulating currents generates torque. This is the operation principle of induction
motors.
• If the current is sinusoidal and the iron core is assumed to have a linear behavior, the magnetic field and the magnetic flux in
the iron core are sinusoidal too.
• Note that the flux is divided in two parts. One flows through the normal (‘pole”) and the other flows through the shaded pole.
These two fluxes are similar in magnitude but different in angle.
Features of the Induction Principle
�Suitable for AC Systems
�The Torque Does Not Vary With Time: No Vibration
�Inherent Rejection of DC Offset: Low Overreach
Solid State Relays
Research Began in the 1940’s. First Commercial Products in the Late 1950’s. Full Development in the 1960’s
Advantages Over Electromechanical Relays
• A solid state relay (SSR) is a solid state electronic component that provides a similar function to an
electromechanical relay but does not have any moving components, increasing long-term reliability.
• Introduction of static relays began in the early 1960’s.
• Their design is based on the use of analogue electronic devices instead of coils and magnets to create the
relay characteristic.
• Early versions used discrete devices such as transistors and diodes in conjunction with resistors,
capacitors, inductors, etc.,
• But advances in electronics enabled the use of linear and digital integrated circuits in later versions for
signal processing and implementation of logic functions..
• Figure shows a small overcurrent relay and the circuit board for a simple static relay.
 Solid State Relay Principle of Operation
• Solid state relays (static relays) are extremely fast in their operation.
• They have no moving parts and have very quick response time and they are very reliable.
• Figure shows the elements used in a single – phase time lag overcurrent relay.
• The AC input from the current transformer CT is rectified and converted to DC voltage Vin through
shunt resistance.

R
CT Rectifier Relay

 A delay time circuit (RC) is used to produce the required time delay.
 If Vin < VR, the base – emitter of transistor TR1 is reversed bias forcing the transistor to be in the cut off state.
 When Vin > VR, transistor TR1 will be in the ON state and in turn will turn on TR2 and the output relay is activated.
 VR is set by R1 and R2.
Computerized Relay
Digital relays
Research Began in the 1960’s. Basic Developments: Early 1970’s. A Technical and Economic Solution: the Microprocessor based
Commercial Relays: Early 1980

• A digital protective relay is a microcomputer controlled relay.

• The data acquisition system collects the transducers information and converts it to the proper form for use by the
microcomputer.
• Information from CT and PT and other systems is amplified and sampled at several kHz. The sampled signals are
digitized with A/D converter and fed to registers in microprocessor system.
• The microprocessor may use some kind of counting technique, or use the Discrete Fourier Transform (DFT) to compare
the information with preset limits for overcurrent , over/under voltage…etc, and then send command through D/A
converter to alarm or trip signals to the circuit breakers.
Operation :
 The relay applies A/D (analog/digital) conversion processes to the incoming the voltages and currents.
 The relay analyzes the A/D converter output to extract the magnitude of the incoming quantity (RMS
value) using Fourier transform concept.
 Further, the Fourier transform is commonly used to extract the signal's phase angle relative to some
reference.
 The digital relay is capable of analyzing whether the relay should trip or restrain from tripping based on
current and/or voltage magnitude (and angle in some applications). Examples of digital relays are shown in
Figure.

Signal Path for Microprocessor Relays

The signal path for voltage and current input signals are shown in Fig.
  After the currents and voltages are reduced to acceptable levels by the instrument transformers, the
signals are filtered with an analog filter
 The signal then digitized and re-filtered with a digital filter.
 Numerical operating quantities are then calculated from the processed waveforms.

Digital Relay Construction

• Analog Input Subsystem
• Discrete Input Subsystem A/D Converter
• Microprocessor
• Discrete output Subsystem
• Operating signaling and communication subsystems
 Other Features include:
 The relay has some form of advanced event recording.
 The event recording would include some means for the user to see the timing of key logic decisions, relay
I/O (input/output) changes, and see in an oscillographic fashion at least the fundamental frequency component
of the incoming AC waveform.
 The relay has an extensive collection of settings, beyond what can be entered via front panel knobs and dials,
and these settings are transferred to the relay via an interface with a PC (personal computer), and this same
PC interface is used to collect event reports from the relay.
 The more modern versions of the digital relay will contain advanced metering and communication protocol
ports, allowing the relay to become a focal point in a SCADA system.
 Advantages of Digital Relays include:
• Low Cost
• Multifunctionality
• Protection and control
• Measurement
• Fault recording
• Communications capability
• Compatibility with Digital Integrated Systems
• High Reliability
• Relays (integration, self-testing)
• Protection system (supervised by the relays)
• Sensitivity and Selectivity
• New Protection Principles
• New Relay Operating Characteristics
• Maintenance-Free
• Reduced Burden on CTs and VTs
• Adaptive Protection
NUMERICAL RELAYS
• The distinction between digital and numerical relay rests on points of fine technical detail, and is
rarely found in areas other than Protection.
• They can be viewed as natural developments of digital relays as a result of advances in
technology.
• Typically, they use a specialized digital signal processor (DSP) as the computational hardware,
together with the associated software tools.

Numerical measurement treatment

I1 U1 numerically the measurement

value is converted into a
D
000101001001
A logical digit and then
compared with another digit
setting value stored 000101001011
in EEPROM stored in a memory
 Advantages of numerical technology
Mode of operation  Comprehensive information supply
Analog Inputs
 clear representation of the fault sequence
Analog-Digital-Conversion • Fault sequence of event and disturbance recording indicate:
• What actually happened ?
yes Fault detection no • What did the current and voltage signals look like (CT saturation) ?
• When did the protection issue a trip signal ?
Protection program Routine program
• How long did the circuit breaker need to operate ?
• What was the magnitude of the interrupted current ?
Command and information output
• How did the system behave after the circuit breaker tripped ?
Methods of Fault Detections

• Magnitude of current – Overcurrent protection

• Magnitude of current in earth and neutral – Earth fault protection

• Magnitude and angle of Impedance (Ratio V/I) Impedance
protection
• Difference between two currents – Differential protection
• Difference between phase angles of two currents – phase
comparison protection
• Magnitude of negative sequence current

• Magnitude of voltage – Overvoltage or undervoltage protection

• Magnitude of frequency – Overvoltage or underfrequency protection

• Temperature – Thermal protection

• Specials i.e. transformer gas protection

THANKS