PSPS Notes.pdf

Protective Relays
Unit I
Pradeep V

Electrical Energy Systems
Electrical Energy is
Generated at few kV and stepped up.
Transmitted through AC and HVDC lines.
Stepped down and distributed at load centers. Its natural mode of
synchronous operation knits the system together
Pradeep V Protective Relays 1 / 33

Causes of Faults
Weather conditions: lighting strikes, heavy rains/ wind/snow
Interference of living beings- tree falling, animal intervention
Equipment failures: short circuit faults due to malfunctioning,
ageing, insulation failure in generators, motors, transformers,
cables
Human errors: forgetting removal of metallic parts after
maintenance and switching the circuit, selecting improper rating of
device

Types of Faults
Shunt fault (short circuit)- high current
line-to-ground
line-to-line
double-line-to-ground
Three phase fault
Series fault (open conductor)- voltage issue
Permanent faults
Transient Faults-momentary tree contact, bird or animal contact,
lightning strike, conductor clashing

Effects of Faults
Low Z
High I
High I square R Loss

Effects of Faults
Voltage and Current for a fault on Transmission line

Effects of Faults
Faults may lead to significant changes in the system quantities-
overcurrent
Over- or Undervoltage
Phase angle
Direction of current flow
Impedance of the current path
System frequency
Temperature
Fault leads to voltage and current signals modulation

Need of Protection
1 To reduce further damage to the equipment
2 To minimize danger to people
3 To reduce fire hazards To reduce stress on other equipment
4 Power quality- voltage sag
5 Interruption duration- associated revenue of the utilities
6 Remove faulted equipment as quickly a to maintain system integrity
and security Maximum Service Continuity with minimum system
disruption

Components of Protection System
1 CT and PT (Sensosrs)
2 Relays
3 Circuit Breaker
4 Communications channels
5 DC supply system
6 Control cables

Trip Circuit

Types of Protection
Apparatus Protection
Transmission Line Protection
Transformer Protection
Generator Protection
Motor Protection
Busbar Protection
System Protection
Out-of-Step Protection
Under-frequency Relays
Islanding Systems
Rate of Change of Frequency Relays

IEEE Standard C37.2-2008
21 - Distance Relay
25 - Synchronizing or Synchronism-Check Device
27 - Undervoltage Relay
32 - Directional Power Relay integrity and security
32 - Directional Power Relay
50 - Instantaneous Overcurrent
51 - AC Inverse Time Overcurrent Relay
50 - Instantaneous Overcurrent
51 - AC Inverse Time Overcurrent Relay
67 - AC Directional Overcurrent Relay
87 - Differential Protective Relay
B –Bus
G – Ground or generator
L—Line
N –Neutral
T – Transformer
U—Unit

Functional Characteristics/ Attributes of
Relaying
To reduce further damage to the equipment
To minimize danger to people
To reduce stress on other equipment
Remove faulted equipment as quickly a to maintain system integrity
and security .

Zones of Protection
Region of power system for which a given relaying scheme is
responsible
A relay should be able to discriminate whether the fault is in its
jurisdiction or not–> secure
This jurisdiction of a relay is called zone of protection.
zone Zone boundary is defined by CT and CB
zones

Zones of Protection
Closed Zone: All apparatus entering the zone is monitored at the
entry . Also called as differential unit, absolute, selective.
Open Zone: limit of zone is not clearly defined. Also called as non
unit, unrestricted or relatively selective . Uncertainty about the
location of boundary
Protection zones are classified into
Primary Protection
Backup Protection or over reached Zone

Primary and Backup Protection
Backup protection are classified as
Local Backup : Common element( transducers, batteries,CB)
Remote Backup : Independent of relays, transducers, batteries, CB
no common failures
Example
Primary R1 and R2 (R2 duplicate for R1)
R9, R10, R4 Remote backup for R1

Zones of Protection
All elements should be encompassed by at least one zone.
Important elements are included in at least two zones
Zones of protection must overlap.
It should Be finite and minimum. faults in overlap zone both zones
operated removing larger section power system

Aspects of Relay Performance
...3S of Performance: Selectivity, Sensitivity and Speed
1 Selectivity( Relay Coordination) : Measure of how a relay element
can differentiate between in zone and out of zone fault
2 Sensitivity : Measure of ability to pickup for in zone faults Smallest
fault and gives relay performance under minimum source condition,
for high resistance fault, low gate faults.
.

Faults Clearing
Relay Data

Speed of Relaying (Faults Clearing)
Relay Data

Speed of Relaying
Speed : Measure of how fast a relay operates for in zone faults
To Minimize sag effects, minimize equipment damage, improve
safety, preserve system stability
The relay should quickly arrive at a decision and circuit breakers
should be fast enough
Intentional Delay for Coordination
Speed Vs. Accuracy Conflict
The consequences of quick tripping decisions are
Nuisance tripping or tripping when there is no fault.
Tripping for faults outside the relay jurisdiction.
High-speed system tend to be less accurate because of lesser
amount of information

Reliability
Degree of certainty that a piece of equipment will perform as intended
Dependability: Measure of certainty that the relays will operate
correctly for all faults for which they are designed to operate
False Trip–> bias towards dependability
Security : Measure of certainty that the relays will not operate
incorrectly for any faults or problems outside the zone or tolerable
transients
Failure to trip–> bias towards security Bias towards to dependable
→ loss of security(misoperations) number of alternative path is
limited or system in emergency state

Reliability
Example
R2 fails to operate → loss of reliability through loss of
dependability
R5 operatre before R2 → unreliable through loss of security

Other Aspects
Economics
Simplicity
Maximizing above factors is a challenge and is an art
Two "knobs" to adjust while setting
1.Sensitivity improves dependability but reduces security
2.Delay shorter delay reduces security
Relay should be fast but not too fast
Relays should be sensitive but not too sensitive

Over current Principle

Selectivity of Overcurrent Relaying

Time Solution
Inverse Time
Instantaneous with constant Delays
Communication Solution(Differential)

Differential Relaying
The Best Protection technique Electrical quantities entering and leaving
the zone is compared If the net between all circuit is zero then no fault
Universally applicable for all parts of power system : Generators,
motors,buses,transformer,lines ,capacitors, reactors etc.,

For normal operations and externals faults(through Faults)
Even with exactly same of CT Iop will not be zero but will be Small(
losses within the zone and differences between the same CTs)

Differential Relaying

Overcurrent Characterisitcs

Overcurrent Characterisitcs 51

Pickup setting of phase overcurrent
relays
Pickup current should be above maximum load current, i.e.; Ipickup
k Imax. k-overload factor – for distribution lines it can be 2, for
transformer, generator it is 1.25-1.5, for motor k=1.05.
Pickup current should be below the minimum fault current i.e;
Ipickup < IFmin.
For setting of pickup current is, kImax Ipickup < Ifmin

Pickup setting of Ground Overcurrent
relays
20 percent -40 percent of the full-load current or minimum
earth-fault current on the part of the system being protected
(Neutral impedance limits the residual current)
Plug Setting Multiplier(PSM) = Secondary current/Relay current
setting
PSM=Primary current during fault, i.e. fault current/Relay current
setting × CT ratio
Time Multiplier Setting :steps in which time can be set

Need of Protection
● Loss of system stability
● Overcurrent,thermal and mechanical damage
● Human Safety
Circuit Breaker
→ Dielectric stress (inversely
proportional to the square of the
distance between the
electrodes)
Fault clearing Time of a circuit Breaker
Relaying time = Time from fault inception to the closure of trip circuit of CB. ( less
than a cycle (numerical relay) to 5 cycles electromechanical relay)
Breaker opening time = Time from closure of the trip circuit to the opening of the
contacts of the circuit breaker.( 1 to 3 cycles)
Arcing time = Time from opening of the contacts of CB to final arc extinction( 1` to
2.5 cycles).
Breaker interrupting time = Breaker opening time + arcing time

Fault clearing time = Relaying time + breaker interrupting time
Arc Voltage
→ Resistive ,
→ magnitude is low about 3
per cent of the rated voltage
→ negative resistance
characteristics

Arc Interruption
i) High resistance Interruption
ii) Cooling
iii) Lengthening,
iv) Constraining
v) Splitting
→ Suitable only for low power circuits
ii) Current Zero Interruption (Need Zero Crossing)
Recovery rate theory
Energy Balance Theory
Circuit Breaker Ratings
Breaking Capacity : Current that CB can interrupt without being destroyed ->RMS
value
Making Capacity: Peak Value

Short Time rating:
Breaking Capacity=√3*V*I*10^6
Voltage across CB contacts
Recovery Voltage : Voltage across CB poles when arc extinguishes
Prospective Current : Current which would have flowin if the breaker did not operated
Transient Recovery Voltage (TRV) or Restriking Voltage:Transient part of recovery
voltage
Transient Recovery Voltage:
The natural frequency of oscillation is given by

Maximum Value of restriking voltage =2* Peak value of system voltage
Rate of rise of recovery voltage → RRRV ….Why?
Maximum value of RRRV →
In view of restriking voltage a short line fault are more dangerous that long line fault
(short with low values of l and C → higher value of omega n)
Resistance Switching
→ to reduce restriking voltage, RRRV
→ Capacitive current or low inductive current

Critical Value of Resistance ( value at which there is no
oscillation)
Frequency of damped oscillations →

Current Chopping
Capacitive Current Interruption

★ Interruption of Capacitive current
★ Problem of high transient voltage
Air Blast Circuit Breaker
★ Current Chopping Phenomenon is the most severe
★ Resistance switching is mainly employed for ABCB
★ Air pressure is maintained at 30 kg/cm2
★ Suitable for high speed repeated operations
★ Most suitable for EHV and UHV applications 400kv range
SF6
★ It is more popular and widely used

★ SF6 gas has 3-5 times better electronegativity(affinity for electrons)) than air
★ It is useful for all voltage applications form 11Kv to 220 KV(famous for HV)
★ SF6 gas has excellent thermal conductivity because of its low molecular
weight
Oil Circuit Breaker
★ Oil has two applications
○ It act as dielectric medium→ 50Kv/cm
○ It act as a cooling medium
★ At 658 K oil decomposed and produces gas of which 70 % is hydrogen
★ OCB has least current chopping phenomenon→ The strength of deionizing
force shall adjust to severity of fault current
★ The chances for arc interruption in the subsequent current zero increased
with OCB but decrease with other types
★ 11kV to 132 KV
★ Volume of oil is more in Bulk OCB because oil is also used for insulation
★ Problems :
○ Carbonisation of live parts
○ Fire hazards
Vacuum Circuit Breaker
★ The vacuum pressure in VCB is maintain around 10^-8 to 10^- 6 torr (1 torr=
1mm of Hg)
★ Principle of arc interruption in VCB is condensation of arc products like cu
vapurs
★ Maintenance is least
★ Suitable for remote and rural applications
★ For interruption of high current at low voltages (3KV to 38KV, 11KV)

Circuit Breaker Rating
● Making Capacity peak value
● Breaking Capacity RMS value
● Short time current rating
● Making capacity =2.55*Breaking capacity
● Making capacity at subtransient
● Breaking capacity at transient
PRACTICE QUESTIONS
1. Which of the following circuit breakers are used for 220 kV substations?
1) Air
2) SF6
3) Vacuum
4) OCB
2. Which of the above circuit breakers can be used in an indoor substation?
a) Air
b) SF6
c) Vacuum
d) OCB
3. A fault occurring on an end-supplied transmission line is more severe from the point
of view of RRRV if it is a
a) long line fault
b) short line fault
c) medium line fault
d) generator fault.
4. In connection with the arc extinction in circuit breaker, resistance switching is
employed wherein a resistance is placed in parallel with the poles of the circuit breaker
as shown in figure. This process introduces damping in the LC circuit. For critical
damping, the value of ‘R’ should be equal to
a)
𝐶
𝐿
b) 0. 5
𝐶
𝐿

c) 0. 5
𝐿
𝐶
d)
1
2π
𝐿
𝐶
5.Rated breaking capacity (MVA) of a circuit breaker is equal to
a) the product of rated breaking current (kA) and rated voltage (kV)
b) the product of rated symmetrical breaking current (kA) and rated voltage
(kV)
c) the product of breaking current (kA) and fault voltage (kV)
d) twice the value of rated current (kA) and rated voltage (kV).
6.If the inductance and capacitance of a system are 1 H and 0.01 µF respectively and
the instantaneous value of current interrupted is 10 amps, the value of shunt resistance
across the breaker for critical damping is
a) 100 kΩ
b) 10 kΩ
c) 5 kΩ
d) 1 kΩ
7.The minimum oil circuit breaker has less volume of oil because
a) There is insulation between contacts
b) The oil between the breaker contacts has greater strength
c) Solid insulation is provided for insulating the contacts from earth
d) None of the above is true
8.The making and breaking currents of a 3 phase ac circuit breakers in power systems
are respectively in ________form.
a) rms value, rms value
b) instantaneous value, rms value
c) rms value, instantaneous value
d) instantaneous value, instantaneous value
9.Resistance switching is normally resorted in case of
a) Bulk oil circuit breakers
b) Minimum oil circuit breakers
c) Air blast circuit breakers
d) All types of breakers
10.If the inductance and capacitance of a system are 1.0 H and 0.01 µF respectively and
the instantaneous value of current interrupted is 10 amp, the voltage across the breaker
contacts will be

a) 50 kV
b) 100 kV
c) 60 kV
d) 57 kV
11.In rural electrification in a country like India with complex network it is preferable to
use:
a) Air break C.B.
b) Oil C.B.
c) Vacuum C.B.
d) M.O. C.B.
12.The current chopping tendency is minimised by using the SF6 gas at relatively:
a) High pressure and low velocity
b) High pressure and high velocity
c) Low pressure and low velocity
d) Low pressure and high velocity.
13.SF6 gas has excellent heat transfer properties because of its:
a) Higher molecular weight
b) Low gaseous viscosity
c) Higher dielectric strength
d) A combination of (a) and (b)
e) A combination of (b) and (c).
14.A fault is more severe from the view point of RRRV if it is a:
a) Short line fault
b) Medium length line fault
c) Long line fault
d) None of the above.
15.The most suitable C.B. for short line fault without switching resistor is:
a) Air blast C.B.
b) M.O.C.B.
c) SF6 Breaker
d) None of the above.
16. Where voltages are low and current to be interrupted is high the breaker preferred is:
a) Air blast C.B.
b) Oil C.B.
c) Vacuum C.B.

d) Any one of the above.
17. The capacitor switching is easily done with:
Air blast circuit breaker
Oil C.B.
Vacuum C.B.
Any one of the above.
18. Which of the following circuit breakers has the lowest voltage range?
a) SF6 circuit breaker
b) Air-blast circuit breaker
c) Tank type oil circuit breaker
d) Air-break circuit breaker
19. The use of high speed circuit breakers
a) Reduces the short circuit current
b) Improves the system stability
c) Decreases the system stability
d) Increases short circuit current
20.The rate of rise of res-striking voltage (RRRV) is dependent upon
Resistance of the system only
Inductance of the system only
Capacitance of the system only
Inductance and capacitance of system
21.The restriking voltage is measured in
a) RMS value
b) Peak value
c) Instantaneous value
d) Average value
22.The making and breaking currents of 3 phase ac circuit breakers in power system are
respectively in what form?
a) Rms value, rms value
b) Instantaneous value, rms value
c) Rms value
d) Instantaneous value, instantaneous value

23.. If the inductance and capacitance of a power system are respectively 1 H and 001
F and the. instantaneous value of interrupted current is 10 A, then the voltage across
the breaker contact will be
a) 50 kV
b) 57 kV
c) 60 kV
d) 100 kV
24.A three-phase circuit breaker is rated at 2000 MVA; 33 kV. Its making current will be
a) 35 kA
b) 70 kA
c) 89 kA
d) 161 kA
25. Rated breaking capacity (MVA) of a circuit breaker is equal to
a) The product of rated breaking current (kA) and rated voltage (kV)
b) The product of rated symmetrical breaking current (kA) and rated voltage
(kV)
c) The product of breaking current (kA) and fault voltage (kV)
d) Twice the value of rated current (kA) and rated voltage (kV)
Overcurrent and Directional
Principle -operates when the measured current exceeds a predetermined threshold
(Pickup current) -either instantaneously or with a delay

Plug Setting Multiplier = Secondary current/Relay current setting
=Primary current during fault, i.e. fault current/Relay current
setting × CT ratio
Time Multiplier Setting :steps in which time can be set
Phase OC relays Vs Ground OC Relays
Residual current for balanced load or three-phase fault Residual current for ground fault
Pickup setting for phase relays → kImax ≤ Ipickup < Ifmin
(k- overload Factor, Transformer, generator 1.25 to
1.5, distribution lines -2, motor =0.5)
Pickup setting for Ground relays → Ipickup≥ 0.3 Irated
(depends or neutral or residual current, low
setting for high voltage lines and higher for rural
feeder)
***Ground relays are more sensitive than phase relays
Note- For pickup setting of (a) phase relays- three phase fault current
(b) ground relays- phase-to-ground fault current
Types based on Time Current Characteristics
Overcurrent relay may generate a trip command either instantly or with a time delay
The time-current characteristic for computation of trip time for overcurrent relay are:
a) Instantaneous relay
b) Time delayed definite time relay
c) Inverse definite minimum time (IDMT) relay
i) Moderately inverse

ii) Very inverse
iii) Extremely inverse
Relay
Coordination
.
Relays are coordinated to avoid the power outage of large area, For a fault , relay
nearest to the fault point should operate first
In case of failure of primary relay, backup relay should operate to remove the faulted
segment.
Methods to achieve correct relay coordination:
i) Current based method
Discrimination by current -fault current varies with the position of the fault -impedance
between the source and the fault
Limitations :
★ It is not practical to distinguish two nearest faults with one feeder length
of much smaller length- further zones are separated by a circuit breaker
which has negligible impedance
★ There would be variations in the source fault level

Discrimination by current -fault current varies with the position of the fault -impedance
between the source and the fault
ii) Time based method
Its operating time is independent of the level of overcurrent - ‘independent definite-time
delay relay’
Limitations :
The longest fault clearance time occurs for faults in the section closest to the
power source, where the fault level (MVA) is highest.
iii) Combination of both time and current based methods
● The relays are set to pickup progressively at higher current levels, towards the
source.
● Time setting is also done in a progressively increasing order towards the source.
Coordination Time Interval (CTI) or Discrimination time : The difference in operating
times of two adjacent relays is kept at 0.3 to 0.5 s.
Ring Main Circuit
The coordination procedure for relays in a ring main circuit is-
● open the ring at the supply point
● coordinate the relays clockwise and then anti-clockwise
The relays looking in a clockwise direction around the ring are arranged to operate in
the sequence RCA, RBC and RAB – fix the settings like radial system
●

The relays looking in the anti-clockwise direction are arranged to operate in the
sequence RBA, RCB and RAC fix the settings like radial system
Ring Mains and Parallel feeder Protection
Practice Questions
1.Consider the protection system shown in the figure below. The circuit breakers
numbered from 1 to 7 are of identical type. A single line to ground fault with zero
fault impedance occurs at the midpoint of the line (at point F), but circuit breaker 4
fails to operate (‘‘Stuck breaker’’). If the relays are coordinated correctly, a valid
sequence of circuit breaker operation is

Answer : (C) 5, 6, 7, 3, 1, 2
2.The over current relays for the line protection and loads connected at the buses
are shown in the figure
The relays are IMDT is natural having the characteristics
top=0.14×time multiplier setting/(plug setting multiplier)0.02 -1
The maximum and minimum fault current at bus B are 2000A and 500A
respectively.Assuming the time multiplier setting and plug setting for relay RB to be
0.1 and 5A respectively,the operating time of RB(in seconds) is ____________.
Ans:0.22
Sol: Assumptions IsetB=1000A(in primary circuit of C.T)
Because IsetB>=Ifl(at bus C)
IsetB=5A (in secondary side of C.T,
So CT Ratio =100/5
Ifmax=2000A ( because minimum operating time is always correspond to maximum
current
Observed by relay-B)
P.S.M = Ifs/C.T ratio×Iset
= 2000/(100/5)×5
top=0.14×0.1/(20)0.02 -1 =0.22
2. A power system with two generators is shown in the figure below. The system
(generators, buses and transmission lines) is protected by six over current relays R_1

to R_6assuming a mix of directional and non-directional relays at appropriate
locations, the remote backup relays for R_4 are
(GATE-16-S2)
a)R_1,R_2 b)R_2,R_6
c)R_2,R_5 d)R_1,R_6
Ans (d)
Soln: Given power system network
According to principles of directional and non-directional over current relays
placement. In given diagram R2, R4, R5 are directional over current relays as shown
below
For the fault on line – 2 R3 and R4 must be operated then R1 and R6 will operated
because R2 and R5 will carry fault current in opposite direction to set
direction.Therefore, Backup for R4 relay are R1 and R6.
Alternator
1. A negative sequence relay is commonly used to protect
(GATE-11)
a)An alternator b)A transformer
c)A transmission line d)A bus bar

Ans: (a)
Soln: negative sequence relay isused to protect the alternator against unbalanced
load conditions.
2. Consider a stator winding of an alternator with an internal high resistance
ground fault. The currents under the fault conditions are as shown in the figure. The
winding is protected using differential current scheme with current transformer of
ratio 400/5A as shown. The current through the operating coil is
(GATE-10)
(a) 0.1875A (b) 0.2A
(c) 0.375A
(d) 60KA
Current through operation coil = 3.125-2.75
= 0.375A
1. Match the items in List-І (Type of transmission line) with the items in List-ІІ
(Type of distance relay preferred) and select the correct answer using the codes
below the lists.
(GATE-09)
List-І List-ІІ
A. Short Line 1. Ohm Relay
B. Medium Line 2.Reactance Relay
C. Long Line 3.Mho Relay
Codes
A B C
(a) 2 1 3
(b) 3 2 1
(c) 1 2 3
(d) 1 3 2
Ans: (a)
Soln:
1.Reactance relays are least affected by are resistance, which are predominating
factor in short line so reactance relays are uses in short lines.

2. Mho relay is less affected by power surges than the ohm relay and reactance
relay, so they are used in the long lines where power surges are predominating
factor.
3. Impedance relay is moderately affected by power surges and arc resistance, so it
is better suited for medium lines.
Ans: ©
3. The transmission line distance protection relay having the property of being
inherently directional is
(A) impedance relay (B) MHO relay (C) OHM relay (D) reactance relay
We know that for different type of transmission line different type of distance relays
are used which are as follows. Short Transmission line -Ohm reactance used
Medium Transmission Line -Reactance relay is used Long Transmission line -Mho
relay is used Hence (C) is correct option.
4. A 3-phase transformer rated 33kV/11kV is connected in delta/star as shown in
figure.The current transformer (CTs) on low and high voltage sides have a ratio of
500/5. Find the currents i1 and i2, if the fault current is 300A as shown in
figure.(GATE 15-S2)
a) i1 = 1/ A, i2=0A
b) i1=0A, i2=0A
c) i1=0A,i2= 1/ A
d) i1=1/ A, i2=1/
Ans : (a)

Sol:
Given Ic =300A∠𝞡A (𝞡 Not given)
By transformer theory Ia=0 IA=0;
Ib=0 IB=0;
And Ic=300A∠𝞡 means
[Note: that reference direction for Ic chosen as leaving its winding at dot and
reference direction for Ic is chosen entering its winding at dot. Hence then, from the
figure the current in line ‘A’ has magnitude of (100/ A, and the current line ‘a’ =0.
These currents are sensed by the CTs. The CTS have turns ratio of 500/5=100
The secondary of the CT on line ‘A’ current 1/100 ×100/ =1/ A,
and the secondary of the CT on line a current of zero.

Generator Protection
complex→ prime mover and its control, excitation system and its control, cooling
system, grid connection, available in variable sizes
** Generator neutrals are never solidly grounded {fault current , asymmetry in RMF
(vibrations)}
** field winding is kept floating with respect to ground
Faults
Abnormal Operating Conditions
Abnormal Conditions does not required automatic tripping
Short Circuits requires fast removal

No hard rule : needs cooperation between protection engineer and the operating and
plant engineer
Tripping of CB to isolate faulty generator is not sufficient
→ Trip field excitation CB ( stored energy dissipation)
→ steam supply has to be bypassed(prime mover)
→ boiler firing has to be stopped
→ uniform cooling (thermal shock)
→ coal supply system has to be stopped
→ Actuates alarm
→ Turns of CO2
4 Dimensions → Generator, Prime mover, Excitation(loss), Grid(Over I, V and F)
Generators can be directly connected to the bus or through a transformer with a breaker
at high voltage( adv. Low current )
Generator Protection-components
(i) Stator protection
(a) Percentage differential protection
(b) Protection against stator inter-turn faults
(c) Stator-overheating protection
(ii) Rotor protection
(a) Field ground-fault protection
(b) Loss of excitation protection
(c) Protection against rotor overheating because of unbalanced three-phase
stator currents
(iii) Miscellaneous
(a) Overvoltage protection
(b) Overspeed protection
(c) Protection against motoring
(d) Protection against vibration
(e) Bearing-overheating protection
(f) Protection against auxiliary failure
(g) Protection against voltage regulator failure
Stator Faults
Smaller→ toroidal CTs
CTs→ same manufacturers, type, size , dedicated for differential scheme(low burden)
Percentage Differential Protection or longitudinal differential protection(?)

→ Complete protection to phase faults
→ 80% to 85% for ground faults(see next section)
→ restraint is low = 10 to 15% , op . current - 0.15 to 0.5 A
External Faults
Internal Fault

Restricted Earth Fault Protection
If fault occurs near to neutral then voltage to drive current is less
If relay is too sensitive then it may operate for CT saturation

p→ percentage winding unprotected
Rn→ Grounding Resistance(chosen to limit If to 10 A to reduce damage)(100 to
200 ohms)
V→ Phase voltage
Third harmonic under Voltage Relay
….3rd harmonic→ design(not pure sinosoid),loads
Interturn Faults
Transverse differential protection
Requires split phase winding (also called as split phase protection)
How to protect for phase faults?
… mostly alaram

Field Ground Fault Protection
Field circuit is isolated from the ground
A second ground fault short circuit windings results in asymmetry in magnetic filed→ vibrations
structural damage

An external voltage source with ground is superimposed → very 1st fault caused current
to flow via relay
Loss of Excitation
Loss of field to main exciter
Poor brush constant
accidental tripping of Field CB
Short Circuits in the field winding
Slip frequency currents in the rotor → heating
If grid can supply reactive power
then the machines will run as induction generator with rotor heating(esp.
Cylindrical rotor) and stator overloading

Else
Under excited ---*** limits in under excited mode
40- field under /overexcitation relay
Rotor Overheating due to Unbalance in stator current
Negative Sequence current → heating
Negative Sequence Relay also act as back up for 87G
I2→ double frequency current in the rotor→ hysteresis and eddy loss is more→ rotor
heating
Thermal characteristics of the machine-- inverse so inverse OC relay can be used
….Negative Sequence overcurrent relay(46)
Speed
High Speed → load rejection, faults
→ governor controls the speed
→ if not need an overspeed protection, over frequency relay or tachnogenerator
at shaft
Low speed → overload(vibrations)

Loss of Prime Mover
Act as synchronous motor→ affects turbine
Current is small as real current is only used to supply losses but direction is reversed
Sensitive directional power relay with MTA of 180 can be used(32)
Over all Generator Protection

By
Pradeep V
Assistant Professor
EEE Department
Alagappa Chettiar
Government College of Engineering & Technology
Karaikudi-03.

Transformer Faults-
Electrically Induced Factors
● lightning surges and switching surges
● Operation under transient or sustained overvoltage conditions – Overheating
● Partial discharge, steep front incident wave → turn to turn faults
Mechanically induced factors:
● abrasion or rupturing of the insulation
● Magnetically Induced electromechanical forces
Thermally Induced Factors:
● Normal heating generated by the loading: degrade the insulation
● Overloading for extended periods of time, through Fault current
● Failure of cooling system.
● Operating a transformer in an over excited condition.
● Operation under excessive t temperature conditions.
..the breakdown of the transformer insulation system.

Transformer Faults-
Mechanical Protection-Sudden Pressure Relay, Gas Analysis etc
Thermal protection-Hot spot temperature, Top oil, etc.
Electrical/Relay protection- differential relay, fuse, overcurrent, earth fault protection

Transformer Protection-Objectives
i)To detect internal transformer faults and abnormal conditions with high sensitivity
(dependability),
➔ internal faults( single turn to turn fault)
➔ Overloads
➔ Overexcitation
➔ through faults.
ii) High degree of security to operation on system faults for which tripping of the
transformer is not required.
➔ Inrush

FAULTS
Winding failures
–turn-to-turn insulation failure
–moisture
–deterioration
–phase-to-phase and ground faults
–external faults (producing insulation failure)
Tap changer failures
-- mechanical
–electrical
–short circuit
–oil leak
–overheating
Bushing failures
–aging, contamination, and cracking
–ﬂashover due to animals
–moisture
–low oil
Core failures
–Core insulation failure
–ground strap burned away loose clamps, bolts,
wedges

Delta Connection
A1 A2
B1
C1
B2
C2
A1
A2
C2
C1
B1
B2

Star Connection
A1 A2
A1
A2
B1
B2
C2
C1

Vector Grouping Star Delta= Yd1
A1
A2
a1
a2
B1
B2
C2
C1
c1
c2
b2
b1

Vector Grouping Star Delta= Yd1
Va
Vab
30 degree leading
Vbc
Vca
Vc
Vb

Differential Protection of Transformer
● CTs on Star side should be connected in Delta
● CTs on Delta side should be connected in Star
● CT ratio should be selected appropriately

Differential Protection of Transformer(note the change of fault type)

Restricted Earth Fault Protection
Winding to core fault→ involves high R, 87
fails to detect
Need a sensitive relay but sensitive only to
internal faults

Restricted Earth Fault Protection Star Side
If there is a Ground fault beyond CTs, no spill current in OCR

Restricted Earth Fault Protection Delta Side
If there is a fault on star side, no spill current in OCR

Inrush Current
During energisation transformer draws 5 to
7 times the rated current-Inrush Current
Characteristics
● Dc offset,
● odd harmonics, and even harmonics.
● composed of unipolar or bipolar
pulses, separated by intervals of very
low current values
● Different in different phases
1 kVA transformer 110 V/220 V, 50 Hz

Inrush Current- Magnitude
● The residual ﬂux in the transformer core
● The point on the wave of the voltage at which the transformer is
energized.
● The magnitude of the source impedance
● The parameters of the transformer including core

When ?
● Energisation of Transformer
● Voltage recovery on fault clearance
● Sympathetic Inrush

Inrush Current- Harmonic Current
2nd and 4th harmonic can be used
to differentiate inrush from fault
★ Either you can block or add to
the restrain current
Sl.No Harmonic Order Magnitude
1 2 75% of
fundamental
2 4 28%
3 5 18%

Cross Phase Blocking Techniques
Typically, only one phase of the differential element exhibits low second-harmonic
Content.
1. Per-phase
2. 2-out-of-3
3. 1-out-of-3
4. Averaging

Over Fluxing
Also called as over excitation
→ Deep saturation
→ Heating with high magnetising current
……….V/F Relay

Incipient Faults
Fault that are not serious at starting but developing
…...Buchcolz relay
Mechanical relay
Gas Acutated Relay
Lies between transformer and the conservator tank

DISTANCE PROTECTION
1
GATE Lecture on
By
Pradeep V
AP/EEE
A.C.G.C.E.T

Overcurrent protection
★ Simple and cheap as non-directional protection does not require
VT.
★ It is not suitable for protection of meshed transmission systems
where selectivity and sensitivity requirements are more stringent.
★ It is not suitable, if fault current and load currents are comparable
2

Effect of source Impedance
4
Length considerations: SIR (Source Impedance Ratio)
➔ Short Lines : SIR > 4
➔ Medium Lines : 0.5 < SIR < 4
➔ Long Lines : SIR < 0.5

Distance Protection
★ More accurate as more information is used for taking decision.
★ Directional, i.e. it responds to the phase angle of current with
respect to voltage phasor.
★ Fast and accurate.
★ Back-up protection. Primarily used in transmission line
protection. Also it can be applied to generator backup, loss of
field and transformer backup protection.
5

Distance Protection...
★ line impedance is directly proportional to the distance to fault
★ Under-impedance relay
★ those faults occurring between the relay and the next sectionalizing
point; and in the other, all other faults.
6

Ideal Characteristics
Z Measured< Z Set → Trips
7
Operating
Time
Measured Impedance

Need of Second Step
Measured Impedance
8
Second Step
Operating
Time

Three Stepped Protection
Zone 1 : 80% of Protected line
Zone 2 : 20% to 50 %(double circuit line) or 50% of shortest line ..delayed one
Zone 3: 120% of Protected line + adjacent longest line …..further delayed

General Torque Equation
❏ T → Net actuating Torque
❏ V,I → voltage and current applied to relay
❏ 𝛗 → Phase between V and I
❏ て → Maximum Torque angle
Appropriate values for the various constants a very wide
range of relay characteristics can be obtained
10
T= K1
I2
+ K2
V2
+ K3
VI sin ( 𝛗 -て ) - K4

Impedance Relay
T= K1
I2
+ K2
V2
+ K3
11
Positive Negative Zero
** over-current relay with voltage restraint.

Impedance relay characteristics
12

R X Diagram
**R–X diagram as a special case of the phasor diagram
13
X
R
Lagging
Load
Leading
Load
Power into
Bus
Power into
Line
0.8 PF
Load
Tr. line
Relay
Location

Impedance relay characteristics
14
V
I
“View the impedance as the voltage phasor, provided that the current is
assumed to be the reference phasor, and of unit magnitude.”-Arun G Padghe

Reactance Relay
T= K1
I2
+ K2
V2
+ K3
15
Positive
Negative
Zero
** over-current relay with Directional restrain.
*** MTA assumed to be 90

Reactance Relay Characteristics
16

Mho Relay
T= K1
I2
+ K2
V2
+ K3
17
Positive
Negative
Zero
★ directional relay with voltage restraint.
★ constant value of mhos at a certain angle and therefore has
been called a mho unit

Mho circle
...circle passing through the origin in the R–X plane, with a diameter of
K3/K2 which makes an angle of maximum torque of −ϕ with the X axis,
18
★ mho unit combines sensitive directional action with accurate ohmic
measurement→ simplicity and reliability.

The Mho Character
Phase faults should operate only on faults involving the phase pair with which
the relay is
associated
19
Source : A. R. van C. WARRINGTON, Application of the Ohm and Mho Principles to Protective Relays

Discussions
Overcurrent Protection.
★ Conventional overcurrent relays will not clear faults during periods of minimum
generation if the short-circuit current then is less than the maximum load
Reactance Relay :
★ Arcing fault tends to prevent the impedance relay from operating.Hence is
indispensable for short lines and for protecting against ground faults.
Mho Relay
★ Long lines, the current may be high enough under load or power swing conditions
★ kv/I miles → minimum line length requirement
20

Effect of Fault Resistance
21
Z Line
Fault
Resistance
Impedance
seen by relay

Effect of Fault Resistance...
Lines too short for the mho unit still must be protected by the reactance ohm unit.
22
Reactance
Relay
Mho
Impedance

Effect of Power Swing
23
Locus of Z during
power swing
Electrical Centre
Mho is relatively insensitive to power swings.

Summary
24
**Source Y.G.Paithankar, Fundamentals of Power System Protection

Communication Assited
25
DUTT: Direct Underreaching Transfer Trip
PUTT: Permissive Underreach
POTT: Permissive Overreach
DCB; Directional Comparison blocking

Practice Questions
Match the items in List-І (Type of transmission line) with the items in List-ІІ
(Type of distance relay preferred) and select the correct answer using the
codes below the lists.
List I List-ІІ
A. Short Line 1. Ohm Relay
B. Medium Line 2.Reactance Relay
C. Long Line 3.Mho Relay
Answer : A-2, B-1, C-3. [GATE-09 2 Mark]
26

Practice Questions
The transmission line distance protection relay having the property of
being inherently directional is
(A) impedance relay
(B) MHO relay
(C) OHM relay
(D) reactance relay
Answer : B.
27

Practice Questions
Reactance relays are normally used for protection against
(A) earth faults
(B) phase fault
(C) Open Circuit faults
(D) None of the above
Answer : A.
28

Practice Questions
In a 3 step distance protection, the reach of the three zones of the relay at
the beginning of the first line typically extend up to
(A) 100% for first line, 50% of second line and 20 % of third line
(B) 80% for first line, 20% of second line and 10 % of third line
(C) 80% for first line, 50% of second line and 20 % of third line
(D) 50% for first line, 50% of second line and 20 % of third line
Answer : C.
29

Practice Questions
Voltage phasors at the two terminal of a transmission line of length 70 km
have a magnitude of 1 pu but 180 out of phase. Assume maximum load
current is ⅕ th of minimum fault current, which one of the following
transmission line protection schemes will not pick up for this condition
(A) Distance protection using Mho set to 80% of line impedance
(B) Directional OC pickup at 1.25 of max load
(C) Pilot relaying with directional comparison
(D) Pilot relaying with segregated phase comparison
Answer : A.
30

UNIT V
PROGRAMMABLE LOGIC CONTROLLERS BASED PROTECTION
Syllabus: Evolution of modern day PLC - Input and Output modules - other functional
elements - Programming the PLC - ladder logic diagram - Boolean language - online and
offline timer programming - communication in PLC - typical applications of PLC – PLC
based protection.
PLC → Special purpose Computer that can work under harsh industrial environments,
mainly used for industrial process controls.

5.1 Evolution of modern day PLC :
5.2 Input and Output Modules :
Sensors sense physical parameters in fields like temperature, pressure, flow etc.,.
The Sensed data is then interfaced with the CPU through interfacing modules called input
and output modules. Input devices can be either start and stop push buttons, switches,
temperature sensor etc and output devices can be an electric heater,motor staters, lights,
valves, relays, etc. I/O module helps to interface input and output devices with a
microprocessor.
Types :
1. Monolithic (“brick”) → fixed amount of I/O capability built into the unit.
2. Modular (“rack”) → individual circuit board “cards” to provide customized
I/O capability.
a. Individual I/O cards may be easily replaced in the event of failure
b. Specific I/O cards may be chosen for custom applications,
c. Some PLCs have the ability to connect to processor-less remote racks
filled with additional I/O cards or modules, thus providing a way to
increase the number of I/O channels beyond the capacity of the base
unit
Parts:
1. Discrete I/O Module (DC)
a. Discrete AC I/o Module
2. Analog I/O Module
Discrete I/O Modules (DC)
A “discrete” data point is one with only two states on and off. Process switches,
push-button switches, limit switches, and proximity switches are all examples of discrete
sensing devices.

In order for a PLC to be aware of a discrete sensor’s state, it must receive a signal
from the sensor through a discrete input channel.
Inside each discrete input module is (typically) a set of light-emitting diodes (LEDs)
which will be energized when the corresponding sensing device turns on.
Light from each LED shines on a photo-sensitive device such as a photo-transistor
inside the module, which in turn activates a bit (a single element of digital data) inside the
PLC’s memory.
This opto-coupled arrangement makes each input channel of a PLC rather rugged,
capable of isolating the sensitive computer circuitry of the PLC from transient voltage
“spikes” and other electrical phenomena capable of causing damage.
Each input channel has its own optocoupler, writing to its own unique memory
register bit inside the PLC’s memory. Discrete input cards for PLCs typically have 4, 8, 16, or
32 channels. In industrial language channels are called points.
In a manner similar to discrete inputs, a PLC connects to any number of different
discrete final control devices through a discrete output channel.
Discrete output modules typically use the same form of opto-isolation to allow the
PLC’s computer circuitry to send electrical power to loads: the internal PLC circuitry
driving an LED which then activates some form of photosensitive switching device
Control wire → single conductor connecting the I/O card channel to the field
device, as opposed to conductors directly common with either the positive or negative
lead of the voltage source
Sourcing Device → A device sending (conventional flow) current out of its control
terminal to some other device(s)
Sinking Device→ a device accepting (conventional flow) current into its control
terminal is said to be sinking current.

If the discrete device connecting to the PLC is not polarity-sensitive, either type of
PLC I/O module will suffice.
For example, the following diagrams show a mechanical limit switch connecting
to a sinking PLC input and to a sourcing PLC input:
On the “sinking” card, the input channel terminal is positive while the common
(“Com”) terminal is negative. and on the “sourcing” card, the input channel terminal is
negative while the common (“VDC”) terminal is positive

Discrete AC Input Module
It takes AC voltage from the field and convert into digital on or OFF
● When pushbutton is pressed, 120V AC is given to the rectification circuit.
● This circuit converts 120V AC into the less level dc voltage which is
delivered to LED of an optical separator.

● The voltage of Zener is set at such a level which can be detected easily.
● When light of led(light-emitting diode) collides with the photo-transistor it
starts working and the position of the pushbutton is transferred in logical
form to the CPU.
Discrete AC Output Module
It uses TRIACs as power switching devices rather than transistors as is
customary with DC discrete output module
● When the CPU sends commands to the energization of the load, then a
voltage is provided to the light-emitting diode of the optical isolator.
● The light-emitting diodes then start illuminating, and by this light
photodiode starts its operation.
● Then this photodiode energizes the triac alternating semiconductor switch
TRIACs and current starts to flow to the output.
● As the triac works in any way, the output to the load is ac.
Analog I/O Module
In the early days of programmable logic controllers, processor speed and memory
were too limited to support anything but discrete (on/off) control functions.
Consequently, the only I/O capability found on early PLCs were discrete in nature.
Modern PLC technology, though, is powerful enough to support the measurement,
processing, and output of analog (continuously variable) signals.
All PLCs are digital devices at heart. Thus, in order to interface with an analog
sensor or control device, some “translation” is necessary between the analog and digital
worlds. Inside every analog input module is an ADC, or Analog-to-Digital Converter, circuit
designed to convert an analog electrical signal into a multi-bit binary word.
Conversely, every analog output module contains a DAC, or Digital-to-Analog
Converter, circuit to convert the PLC’s digital command words into analog electrical
quantities
Analog I/O is commonly available for modular PLCs for many different analog
signal types, including:
● Voltage (0 to 10 volt, 0 to 5 volt)
● Current (0 to 20 mA, 4 to 20 mA)

● Thermocouple (mV)
● RTD
● Strain gauge
As PLCs typically use 16-bit signed binary processors, the integer values are limited
between -32,768 and +32,767(2^16). Analog signals can use voltage or current with a
magnitude proportional to the value of the process signal. For example, an analog 0 to 10 V
or 4-20 mA input would be converted into an integer value of 0 to 32767. The
programming inside the PLC will use a SCL or scaling function to take this 0-32767 value
and transpose it into the desired units of the process so the operator or program can read
it.
Current inputs are less sensitive to electrical noise (e.g. from welders or electric
motor starts) than voltage inputs. Distance from the device and the controller is also a
concern as the maximum travelling distance of a good quality 0-10V signal is very short
compared to the 4-20mA signal. The 4-20mA signal can also report if the wire is
disconnected along the path as it would return a 0mA signal and a fault can be reported.
In fact, the PLC can’t even measure current. So what happens is that inside the analog
input module, a resistor is put between positive (AI) and negative (AGND). This will not
only make up the closed loop but also converts our current signal to a voltage signal.
Fig: Analog I/O Modules
SPECIAL I/O Modules:
1 . High-Speed Counter Module

❖ The high-speed counter module is used to provide an interface for applications
requiring counter speeds that surpass the capability of the PLC ladder program.
❖ They have the electronics needed to count independently of the processor.
❖ A typical count rate available is 0 to 75 kHz. which means the module would be
able to count 75.000 pulses per second.
2. Stepper·Motor Module
❖ The stepper-motor module provides pulse trains to a stepper-motor translator.
which enables control of a stepper motor.
❖ The commands for the module are determined by the control program in the PLC
3. BASIC or ASCD Module
❖ The ASCII module allows the transmitting and receiving of ASCII fi les. These files
are usually programs or manufacturing data.
❖ The modules are normally programmed with BASIC commands.
Intelligent I/O → they have their own microprocessors on board that can function in
parallel with the PLC
Eg: PID Module
A PID module allows process control to take place outside the CPU. This
arrangement prevents the CPU from being burdened with complex calculations. The
microprocessor in the PID module processes data. compares the data to set points
provided by the CPU. and determines the appropriate output signal.
The CPU
It includes memory module(s), communications circuitry. and power supply
Power Supply
Provides all the voltage levels required for operation. The power supply converts
120 or 220 V ac into the dc voltage required by the CPU, memory, and 110 electronic

circuitry. The PLC operates on +5 and - 5 V dc. Therefore, the power supply must be
capable of rectifying the stepping-down of the ac input voltage to a usable level of dc
voltage.
Processor
The term CPU is often used interchangeably with the term processor. However. by
strict definition, the CPU term encompasses all the necessary elements that form the
intelligence of the system. There are definite relationships between the sections that form
the CPU and the constant interaction among them. The processor is continually
interacting with the system memory to interpret and execute the user program that
controls the machine or process. The system power supply provides all the necessary
voltage levels to ensure proper operation of all processor and memory components. The
CPU contains the same type of microprocessor found in a personal computer. The
difference is that the program used with the microprocessor is designed to facilitate
industrial control rather than provide general purpose computing. The CPU executes the
operating system. manages memory. monitors inputs, evaluates the user logic (ladder
program), and turns on the appropriate outputs.
Modes of Operation.
RUN Position
• Places the processor in the Run mode
• Executes the ladder program and ener· gizes output devices
• Prevents you from performing online program editing in this position
• Prevents you from using a programmer! operator interface device to change the
processor mode Front view
PROG Position
• Places the processor in the Program mode
• Prevents the processor from scanning or executing the ladder program. and the
controller outputs are de-energized
• Allows you to perform program entry and editing
• Prevents you from using a programmer! operator interface device to change the
processor mode REM Position
• Places the processor in the Remote mode:
REMote Program. or REMote Test mode
• Allows you to change the processor mode from a programmer/operator interface
device
• Allows you to perform online program editing
Memory
The memory of a PLC is broken into sections that have specific functions. Sections
of memory used to store the status of inputs and outputs are called input status files or
tables and output status files or tables . These terms simply refer to a location where the
status of an input or output device is stored. Each bit is either a 1 or 0, depending on
whether the input is open or closed.
Timer files are usually three words long. One word contains timer status
information; another contains the preset value or set point: and the last contains the
accumulated value. Counter files. also three words long. have the same configuration as

the timer. Bit. control, and integer files are also used to allow more programming flexibility
and to allow for more complex instructions
Volatile memory → stored information if all operating power is lost or removed. Volatile
memory is easily altered and is quite suitable for most applications when supported by
battery backup
Nonvolatile memory → retain stored information when power is removed accidentally or
intentionally.
Types :
★ Read-Only Memory (ROM)
★ Random Access Memory (RAM or R/W)
★ Programmable Read-Only Memory (PROM)
★ Electrically Erasable Programmable Read-Only Memory (EEPROM)
★ Erasable Programmable Read-Only Memory (EPROM)
Timers
The main function of a timer is to keep an output on for a specific length of time. A
good example of this is a garage light, where you want power to be cut off after 2 minutes
so as to give someone time to go into the house.
The working of the timer circuit is based on the four main parts. These are as
follows.
● Input Signal
● Internal Power Supply
● Digital Timer Display
● Output Signal
Each of the Internal parts of the timer circuit has various features and functions.
Timer Auxiliary Power Supply (APS): The input power supply is provided for the
proper functioning of the timer circuit. This can be connected with the AC or DC supply
like 230V AC or 5/10 V DC.
Timer Start or Set Operation Signal:If the auxiliary power supply is ‘on’, the timer
will give the momentary input pulse for the given circuit.
Reset Timer Signal: The device or other systems can be reset by switching the APS
in the ‘on’ or ‘off’ condition.
Output Function: There are multiple output functions. It helps to select the proper
functions for the applications. The output gets activated as an output signal of the timer
circuit.
Timer Display: The digital timer displays the set and elapsed timing value. For the
automation purpose, the values can be displayed in a few milliseconds (ms). This will be
easy for tracking your automation system.

The three different types of timers that are commonly used are a Delay-OFF, a
Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer activates immediately when
turned on, and will start counting down from a programmed time before cutting off once
the enabling input is off. A Delay-ON timer is activated by input and starts accumulating
time, counts up to a programmed time before cutting off, and is cleared when the enabling
input is turned off. A Delay-ON-Retentive timer is activated by input and starts
accumulating time, retains the accumulated value even if the (ladder-logic) rung goes
false, and can be reset only by a RESET command. Visible details for the rest of the
program to use could include:
Counters are primarily used for counting items such as cans going into a box on an
assembly line. This is important because once something is filled to its max the item
needs to be moved on so something else can be filled. Many companies use counters in
PLC's to count boxes, count how many feet of something is covered, or to count how many
pallets are on a truck.
There are three types of counters, Up counters, Down counters, and Up/Down
counters. Up counters count up to the preset value, turn on the CTU (CounT Up output)
when the preset value is reached, and are cleared upon receiving a reset. Down counters
count down from a preset value, turns on the CTD (CounT Down output) when 0 is
reached, and are cleared upon reset. Up/Down counters count up on CU, count down on
CD, turn on CTUD (CounT Up/Down output) when the preset value is reached, and cleared
on reset
Programming the PLC
The PLC programming is an important task of designing and implementing control
application depending on customers need. A PLC program consists of a set of instructions

either in textual or graphical form, which represents the logic to be implemented for
specific industrial realtime applications.
A dedicated PLC programming software comes from a PLC hardware of specific
manufacturer that allows entry and development of user application code, which can be
finally download to the PLC hardware. This software also ensures Human Machine
Interface (HMI) as a graphical representation of variables. Once this program gets
downloaded to the PLC and if the PLC is placed in Run mode, then the PLC continuously
works according to the program.
A CPU of the PLC executes two different programs:
1. The Operating System
2. The User Program
The Operating System
The operating system organizes all the functions, operations and sequences of the
CPU that are not associated with a control task. The OS tasks include
● Handling a hot restart and warm restart
● Updating and outputting the process image tables of input and outputs
● Executing the user program
● Detecting and calling the interrupts
● Managing the memory areas
● Establishing communication with programmable devices
The User Program
It is a combination of various functions which are required to process an
automated task. This must be created by the users and need to be downloaded to the CPU
of the PLC. Some of the tasks of the user program include:
● Initiating all the conditions for starting the specified task
● Reading and evaluating all binary and analog input signals
● Specifying output signals to all binary and analog output signals
● Executing interrupts and handling errors
In present industrial automation sector, there are several leading PLC
manufactures that develop typical PLC’s ranging from small to high-end PLC’s. Each and
every PLC manufacturer has its own dedicated software to program and configure the PLC
hardware. But the PLC programming language is varied depending on the manufacturers.
Some manufacturers have common programming languages and some others have
dissimilar. Some of the standard programming languages of PLC are basically of two
types, which are further sub-divided into several types, which are as follows:
1.Textual language
● Instructions List (IL)
● Structured Text (ST)
2. Graphical language
● Ladder Diagrams (LD)
● Function Block Diagram (FBD)
● Sequential Function Chart (SFC)

Compared with text based languages, graphical languages are preferred by many
users to program a PLC due to their simple and convenient programming features. All the
necessary functions and functional blocks are available in the standard library of each
PLC software. These function blocks include timers, counters, strings, comparators,
numeric, arithmetic, bit-shift, calling functions, and so on.
PLC Programming Devices
Various types of programming devices are used to enter, modify and troubleshoot a
PLC program. These programming terminal devices include handheld and PC based
devices. In the handheld programming device method, a proprietary device is connected
to PLC through a connecting cable. This device consists of a set of keys that allows to
enter, edit and dump the code into the PLC. These handheld devices consist of small
display to make the instruction that has been programmed visible. These are compact and
easy to use devices, but these handheld devices have limited capabilities
Most popularly a Personal Computer (PC) is used for programming the PLC in
conjunction with the software given by the manufacturer. By using this PC we can run the
program in either online or offline mode, and can also edit, monitor, diagnose and
troubleshoot the program of the PLC
Ladder Logic diagram
This language was invented for the express purpose of making PLC programming
feel “natural” to electricians familiar with relay-based logic and control circuits. While
Ladder Diagram programming has many shortcomings, it remains extremely popular in
industries automation.
Every Ladder Diagram program is arranged to resemble an electrical diagram,
making this a graphical (rather than text-based) programming language.
Ladder diagrams are to be thought of as virtual circuits, where virtual “power”
ﬂows through virtual “contacts” (when closed) to energize virtual “relay coils” to perform
logical functions.
None of the contacts or coils seen in a Ladder Diagram PLC program are real; rather,
they act on bits in the PLC’s memory, the logical interrelationships between those bits
expressed in the form of a diagram resembling a circuit. being edited on a personal
compute
● Contacts appear just as they would in an electrical relay logic diagram – as short
vertical line segments separated by a horizontal space.
● Rung input : checkers (contacts)
○ —[ ]— Normally open contact, closed whenever its corresponding coil or an
input which controls it is energized. (Open contact at rest)

○ —[]— Normally closed ("not") contact, closed whenever its corresponding
coil or an input which controls it is not energized. (Closed contact at rest)
● Rung output: actuators (coils)
○ —( )— Normally inactive coil, energized whenever its rung is closed.
(Inactive at rest)
○ —()— Normally active ("not") coil, energized whenever its rung is open.
(Active at rest)
● Each horizontal line is referred to as a rung, just as each horizontal step on a
stepladder is called a “rung.” Extreme vertical lines are called as rails
Figure :Ladder Logic diagram for Or Gate and AND Gate
Boolean language:-
Some PLC manufacturers use Boolean language, also
called Boolean mnemonics, to program a controller. The
Boolean language uses Boolean algebra syntax to enter and
explain the control logic. That is, it uses the AND, OR, and
NOT logic functions to implement the control circuits in the
control program

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PSPS Notes.pdf