Power systems Protection

course

Department of Electrical Energy Engineering Dr Audih 1

Al-Balqa Applied University

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Part 5

Relays

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Relay

Is a device which receive a signal from the power

system thought CT and VT and determines whether

conditions are "normal" or "abnormal" ,if an abnormal

condition detect, relay signals CB to trip equipment .

The Purpose of the Relay:

Relaying is used to isolate only the faulty component of

power system.

Relay is classified into two groups:

1. Primary relaying equipment..

2. Back-up relaying equipment: works only when the

primary relaying equipment fails .

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Relay and back-up relay

back-up

relay

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**Note: Backup relay is with delay time greater than main relay

Discrimination Characteristics of Relay

1)Speed: (1/60 sec):

• Minimizes damage from current.

• Maximizes power transfer during normal conditions.

2)Security: Should not cause circuit breaker to open

during normal conditions.

3)Dependability: Should cause circuit breaker to open

during abnormal conditions.

4)Sensitivity: Ability to detect all faults for the expected

limiting system and fault conditions

5)Selectivity: Ability to discriminate between faults

internal and external to its intended protective zones.

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Classifications of Relays

Protection relays can be classified accordance with:

a) Function of Relays: Auxiliary relay, protection relay,

monitoring relay, control relay

b) Construction of Relays: Electromagnetic relay, Solid

state relay, microprocessor relay, computerized relay

and nonelectric (thermal, pressure..etc.)relay.

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c) Incoming signal of Relays:- Current, voltage,

frequency. Temperature, pressure, velocity, and

others.

d) Type of protection: Over current, directional over

current, distance, over voltage, differential and others.

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1.Electromagnetic relays: The principle is a mechanical

force causing operation of a relay contact. The

mechanical force provided from magnetic core

windings). The advantages are simply, cheap and

reliable .

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Relay construction types

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Electromechanical relays types

Electromagnetic relays construction:

Electrical magnetic

Mechanical components.

Have an operating coil and various contacts and are

very strong, inexpensive and reliable, they required

maintenance by skilled personnel.

Their types are:

magnetic attracted armature relays.

magnetic induction relays.

moving coil.

Thermal.

motor operated.

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1.1. Magnetic attraction relays

Magnetic attraction relays :

Its supplied by AC or DC,

operate by the movement of metal contacts when it is

attracted by the magnetic field produced in the coil.

There are two types:-

a) Attracted armature (clapper type) Figure 1.

Figure 1

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Attracted armature principle:

• The armature is attracted to

the electromagnet when

the current reaches a

certain predetermined

value and force of the

armature will trip .

• The armature is attracted

against gravity or a spring.

Setting can be carried by :

adjusting the distance of the armature from the

electromagnet,

the tension of the spring,

The armature carries the moving part of the contact, which is

closed or opened according to the design.

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b) Plunger type relay:

•Its other type of magnetic attraction relays which consist

of a piston or solenoid relay, Figure 2,

• piston is attracted axially within the field of the

solenoid, the piston carries the operating contacts. This

called plunger type relay.

Figure 2 Solenoid-type (plunger ) relay

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And reluctance R=mmf/φ

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Relay setting operation depending on :-

1. Tension of the spring .

2. Resistance of the solenoid.

Attraction relays effectively have no time delay and, for

that reason, are widely used when instantaneous

operations are required.

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1.2 Magnetic induction relays works only with alternating current.

They are three classes types :-

It consists of two electromagnets

constructed as shown in Fig. one for

voltage and the other for current, which

operates on a moving conductor, in the

form of a disc.

Fig.. Induction type overload relay

a) Wattmetric-induction type relay

Induction working Principle

• Induction type relay:

General operating principle:

The two magnets of the induction type

relay produce two alternating magnetic

fields

primary coil

Secondary coil

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b) Induction Type Relay with plug settings

Fig. Induction type relay with Plug settings

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c) Induction-Cup relay

Operation as induction disc.

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. Dr Audih

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1.3 Shaded-pole relay

operation of the electromagnetic section is short-

circuited by means of a copper ring or coil.

creates a flux in the area influenced by the short

circuited section

(the so-called shaded section) which lags the flux in the

non-shaded section, see Figure .

> Note that the main coils has TAPS, this means that the number of turns

is actually adjustable. Dr Audih

In the electromagnetic induction principle, the relay

element has a non­magnetic rotor (an aluminum or

copper disc or cylinder)

coils create magnetic fluxes that induce circulating

currents which generates torque (like operation

principle of motor)

the flux is divided in two parts:-

a) One flows through the normal ('pole") and

b) 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.

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2. Solid State Relays (SSR)

is a solid state electronic component that a similar function

to an electromechanical relay but does not have any moving

components.

Their design is based on analogue electronic such as

transistors and diodes ,resistors, capacitors, inductors, etc.,

Advances is enabled the use of linear and digital integrated

circuits for signal processing and implementation of logic

functions.

A small over-current relay and the circuit board for a simple

static relay are shown in figure.

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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.

Fig. 10

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3. Computerized Relay

3.1. Digital relays

Utilizes a microcontroller with software based

protection algorithms for the detection of electrical

faults.

They are also called numerical relays.

Microprocessors and microcontrollers replaced

analogue circuits used in static relays to implement

relay functions.

Digital relays introduce analog or digital (A/D) of all

measured analogue quantities and use a

microprocessor to implement the protection algorithm.

Digital relay contain advanced metering and

communication protocol ports, allowing the relay to

become a focal point in a SCADA system.

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. Fig. Digital relays

Examples of digital relays are shown in Figure

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Signal Path for Microprocessor Relays

The signal path for voltage and current input signals are

shown in Fig..

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Digital Relay Construction

Analog Input Subsystem

Input Subsystem A/D Converter

Microprocessor

Discrete output Subsystem

Operating signaling and communication subsystems

Fig.13

Digital Relay Architecture

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Analog Filter to reduce noise

Sampling signal for analysis

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2-

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Finally

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The relay logic makes the final decisions for tripping and other

relay functions.

In some modern relays, the logic can be programmed by the user.

The results of a logic function could be used to modify the

protection method.

Sometimes this is called "torque control" to make a similitude with

old electromechanical relays.

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Programmable logic :

is an extraordinary feature of digital relays. The user has a large

number of possibilities to define a logic function according to the

present needs.

Inside the relay, there is a logic variable associated with several

relay and protection functions.

These variables can be employed to create new logic variables,

according to laws defined by the user. Dr Audih

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Advantages of Digital Relays

Low Cost

Multi functionality

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 Dr Audih

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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.

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Advantages of numerical technology

All information supply

clear representation of the fault sequence

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Protection Methods

Overcurrent (50, 51)

Voltage (59, 27)

Directional (67)

Distance (21)

Differential (87)

Frequency (81)

Phasor calculation

Protection methods

Relay logic

These routines implement the protection function:

overcurrent, directional, distance, differential, etc. as

ANSI device numbers marked in appendix.

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Appendix

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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.

Pickup 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..

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.

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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 ofthe 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

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ANSI Device Numbers

The ANSI Standard Device Numbers denote what features a

protective device supports (such as a relay or circuit breaker).

These types of devices protect electrical systems and components

from damage when an unwanted event occurs, such as a electrical

fault.

List of Device Numbers

1 - Master Element

2 - Time Delay Starting or Closing

Relay

3 - Checking or Interlocking Relay

4 - Master Contactor

5 - Stopping Device

6 - Starting Circuit Breaker

7 - Anode Circuit Breaker

8 - Control Power Disconnecting

Device

9 - Reversing Device

10 - Unit Sequence Switch

12 – Over speed Device

13 - Synchronous-speed Device

14 – Under speed Device

15 - Speed - or Frequency-Matching

Device

16 - Elect. operated valve (solenoid

valve)

17 - Distance Relay

18 - Temperature Control Device

19 - Synchronizing or Synchronism-

Check Device

20 - Apparatus Thermal Device Dr Audih

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21 - Isolating Contactor

22 - Annunciator Relay

23 - Directional Power Relay

24 - Polarity or Polarizing Voltage

Devices

25 - Undercurrent or Under power

Relay

26 - Bearing Protective Device

27 - Mechanical Conduction Monitor

28 - Field Relay

29 - Field Circuit Breaker

30 - Running Circuit Breaker

31 - Manual Transfer or Selector

Device

32 - Reverse-phase or Phase-Balance

Relay

33 - Phase-Sequence Voltage Relay

34 - Incomplete-Sequence Relay

35 - Machine or Transformer Thermal

Relay

36 - Instantaneous Overcurrent

37 - AC Time Overcurrent Relay

38 - AC Circuit Breaker

39 - Exciter or DC Generator Relay

40 - High-Speed DC Circuit Breaker

41 - Power Factor Relay

42 - Field Application Relay

43 - Overvoltage Relay

44 - Voltage or Current Balance Relay

45 - Machine Split Phase Current Balance

46 - Time-Delay Stopping or Opening Relay

47 - Pressure Switch

48 - Ground Detector Relay

49 – Governor

50 - Starts per Hour

51 - AC Directional Overcurrent Relay

52 - Blocking Relay

53 - Permissive Control Device

54 - Level Switch

55 - DC Circuit Breaker

56 - Alarm Relay

66 - Position Changing Mechanism

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67 - DC Overcurrent Relay

68 - Phase-Angle Measuring or Out-of-Step

Protective Relay

69 - AC-Reclosing Relay

70 - Frequency Relay

71 - Automatic Selective Control or Transfer Relay

72 - Operating Mechanism

73 - Carrier or Pilot-Wire Receiver Relay

74 - Lockout Relay

75 - Differential Protective Relay

76 - Line Switch

77 - Regulating Device

78 - Voltage Directional Relay

79 - Voltage and Power Directional Relay

80 - Tripping or Trip-Free Relay

81 - Reluctance Torque Synchrocheck

82 - Autoloading Relay

Dr Audih