This document provides an overview of advanced power system protection topics including definitions, objectives of power system protection, relay characteristics, methods of discrimination, distance protection, current balance protection, phase comparison protection, fault detection techniques using zero sequence systems, sequence filters, general relay equations, fuses, and time-current characteristics. It discusses concepts such as discrimination, stability, sensitivity, repeatability and provides examples to illustrate relay equations and characteristics.

1. ECNG 6503 -1
Advanced Power Systems Practice
Lecturer
Prof Chandrabhan Sharma
University of the West Indies
Trinidad and Tobago

2. TABLE OF CONTENTS
1. GENERAL RELAY EQUATIONS / ELECTROMAGNETIC RELAYS
2. FUSES
3. OVERCURRENT PROTECTION
4. DIFFERENTIAL / PILOT WIRE PROTECTION
5. TRANSFORMER / REACTOR PROTECTION
6A. GENERATOR PROTECTION
6B. STATION BUS PROTECTION
7. DISTANCE PROTECTION
8. STATIC RELAY
9. DIGITAL TECHNIQUES / SAMPLING
10.POWER LINE CARRIER
11.APPENDIX

3. ECNG 6503
Advanced Power System Protection
DEFINITIONS :-
1. Discrimination
(a) Absolute Discrimination
- Applied to unit systems
- Responds to within zone only
(b) Dependent (Relative) Discrimination
- Applied to non-unit systems
- Coordinated responses of a number of
similar systems.

4. DEFINITIONS:- (cont’d)
2. Stability
- unit systems
- remains inoperative until fault….
3. Sensitivity
- level of monitored signal
4. Repeatability
- consistency in repeated tries

5. OBJECTIVE OF POWER SYSTEM
PROTECTION
- Detect and isolate faults instantaneously.
- Isolate the minimum number of circuits.
- Restore the system to normal configuration ASAP.
(self-clearing faults – Auto reclosure)
- Discriminate between normal and abnormal
conditions (e.g.: load current, overload and fault)

6. CHARACTERISTICS OF RELAYS
1. Reliability must act when called on. Relay is idle most time.
2. Selectivity differentiate between normal and abnormal
3. Sensitivity discrimination
4. Speed
5. Instantaneous

7. METHODS OF DISCRIMINATIONS
1. By Time
* Each relay set to operate for I > 2000A (say)
If IF > 2000A
Breakers A, B, & C will trip.

8. Hence introduce time delay(t) s.t.
Station (t)
D 0
C 0.4 ѕ
B 0.8 ѕ time discrimination
A 1.2 ѕ
Disadvantage:
Fault currents < 2000A will not be interrupted.

9. 2. By Current Magnitude
Uses the fact that for a radial feeder, at unity
voltage, fault current increases as you approach
the generator.
Disadvantage – Backup cannot be properly done
3. Combination of Time and Current
Best of both systems

10. CO-ORDINATION OF RING MAIN
Directional Relays

11. DISTANCE PROTECTION
ƵAB , ƵBC - feeder impedances
Ƶ1 , Ƶ2 - Zone 1 and Zone 2 reach
[Note : Ƶ1 ≈ (80 – 85%) ƵAB]

12. CURRENT BALANCE (Unit Protection)

13. OPPOSED VOLTAGE PROTECTION
(Radial Feeders)

14. PHASE COMPARISON METHOD:
When communication circuits are unable to provide
faithful transmission of amplitude information, phase
angle comparison is done i.e.:
Phase-comparison carrier current protection
Alternate half cycles of current from either end are
compared c a locally derived signal.
no fault → displacement = 0˚
fault → displacement = 180˚

15. DETECTION
Discriminating between Fault Types.
1. Zero Sequence Systems :-
(a) System Neutral
* Can only be used close to neutral
* 3rd harmonic can be a problem

16. (b) Core balance transformer
Output only obtained if zero sequence current exists.

17. (c) Residual connection of line C.T.s

18. (d) Summation transformer
Derivation of a representative 1 quantity from a 3 system.
Fault Type O/P(n=1)
R-E 5 IF
Y-E 4 IF
B-E 3 IF
R-Y or V-B IF
R-B 2 IF
R-Y-B or R-Y-B-E √3 IF

19. SEQUENCE FILTERS
Ƶ0 Current Filter:
I0 = 1/3 [Ia + Ib + Ic]
Current to relay = 3 I0
= Ia + Ib + Ic

20. Ƶ0 Voltage Filter:
V0 = 1/3 [Va + Vb + Vc]
∴ 3V0 = Va + Vb + Vc

21. NEGATIVE SEQUENCE FILTER
1. Voltage Filter: Vop = V2
R adjusted until VOP is equal ≡ X/√3 = R
VOP is now purely negative sequence.

22. 2. Current Filter

23. GENERAL RELAY EQUATIONS
All relays are comparators, either:-
(a) Amplitude
or
(b) Phase
Amplitude Comparator
Two quantities are opposed and relay operates when
operating quantity exceeds the magnitude of the
restraining quantity irrespective of phase relationship.

24. An inherent phase comparator operates when one
input quantity has a defined phase relationship with
the other irrespective of magnitude.
An inherent amplitude comparator acts like a
phase comparator if the i/p quantities are changed to
the sum and difference of the two original quantities.
(vice versa)
e.g. If Amplitude Comparator operates on A B
then A B A - B is only true for a defined phase relationsh ip

25. Relay R is at threshold. A & B supplied to R in any arbitrary
combination.
Using ‘A’ as the reference vector:
i/p # 1 k1 A k 2 B [cos( - θ) j sin( - θ)
i/p # 2 k3 A k 4 B [cos( - θ) j sin( - θ)
Where k1, k2, k3 and k4 are design constants.

26. AMPLITUDE COMPARATOR
At threshold, the moduli of both inputs would be equal
irrespective of phase angle.
The locus of the moduli will yield the relay characteristic:
[k1 A k 2 B cos( - θ)]2 [k 2 B sin( - θ)]2
[k3 A k 4 B cos( - θ)]2 [k 4 B sin( - θ)]2
Re - arranging terms
2 2
2 2
(k1 k 3 ) A 2(k1k 2 k 3k 4 ) A B cos( - θ) (k 2 k 2 ) B
2 4 0
2
Divide acrossby (k2 k 2 ) A
2 4
2 2 2
B (k k k 3 k 4 ) B k1 k 3
2 1 22 2
cos( - θ) 0
A k2 k4 A k2 k2
2 4

27. Rearranging in the form of an equation of a circle in the complex
plane:
2
B B
2ζ cos( - θ) ζ r2
A A
Comparing coefficients :-
2 2
k1k 2 k 3k 4 k1 k 3
ζ ζ -r
2 2
k2 k2
2 4 k2 k2
2 4
2 2 2
2 k 1k 2 k 3 k 4 k 1 k 3 2 k 1k 4 k 2 k 3
r r
k2 k2
2 4 k 2
2 k 2
4 k2 k2
2 4

28. This represents a circle with:
k1k 4 k 2 k 3
radius r
k2 k2
2 4
k1k 2 k 3k 4
centrec - ζ θ '
θ'
k2 k2
2 4
i.e. Coordinates = - [cos + j sin ]

29. PHASE COMPARATOR
Let the input be :
i/p # 1 k1 A k 2 B [cos( - θ) j sin( - θ)
i/p # 2 k3 A k 4 B [cos( - θ) j sin( - θ)
For phase comparison, relay will only operate when product of i/p is positive.
Let = phase angle of i/p #1
Let β = phase angle of i/p # 2
∴ for threshold ( - β) = 90º
∴ for threshold tan( - β ) =
tan α - tan β
1 tan α tan β

30. i.e.1 tan α tan β 0
1
Or tan α - .......... .......... .......(1)
tan β
Imag k 2 B sin( - θ)
For i/p # 1 tanα
Real k1 A k 2 B cos( - θ)
k 4 B sin( - θ)
For i/p # 2 an β
t
k3 A k 4 B cos( - θ)
Sub into (1)
2
k 2 k 4 B sin ( - θ) -k1k 3 A
2
k1k 4 A B cos( - θ)
2
k 2 k 3 A B cos( - θ) - k 2 k 4 B
2
Divide acrossby k 2 k 4 A
2
B k1k 4 k 2 k 3 B k1k 3
cos( - θ) 0
A k 2k 4 A k 2k 4

32. Example
Given a percentage differential relay which operates when the
difference of the current entering the circuits exceeds 5% the sum
of these currents or 10% of the mean through current.
Determine the characteristic equation for the relay.
Solution
The relay operates when on current is 10% > than the other, i.e.
relay is said to have a 10% bias.

33. Relay operates when:
I1-I2 > s{(I1+I2)/2} where s =10%
Let operating winding be supplied with (I1-I2)
Let restraining winding be supplied with s{(I1+I2)/2}
∴ I1-I2 = k1⃒I1⃒ + k2⃒I2⃒(cos +j sin )
s/ (I +I ) = k3⃒I1⃒ + k4⃒I2⃒(cos +j sin )
2 1 2
where is angle between I1 and I2 ( =0)
by comparing coeffs k1 = - k2 = 1
k3= k4 = s/2

34. For an amplitude comparator:
For s = 0.1 → r = 0.1004 and c = 1.004
∴ equation for amplitude comparator is

35. E.g.
Determine the values of k1, k2, k3, k4 for an inherent phase comparator
which will act like the amplitude comparator given before.
For phase comparator :
&
∴ Comparing coeffs:
k1 k4 - k2 k3 = s …………………⑴
k1 k4 + k2 k3 = - {1 + (s/2 )2} …………………⑵
2k2 k4 = 1 - (s/2 )2 …………………⑶
⑴+ ⑵ → 2k1 k4 = - (s/2 ) 2 + s - 1 = -[(s/2 ) – 1]2

36. and 2k2 k4 = - {(s/2) + 1} {(s/2) – 1}
∴ k1 = - 2 k4 = (s/2 ) – 1
k2 = - 2 k3 = (s/2 ) + 1
∴If we incorporate (-2) into k4 and k3 we have
* Amplitude not important for phase comparator

37. ∴ Inputs are:
No. 1. [ I1 {(s/2 ) – 1} + I2 {(s/2 ) + 1}] ⒜
No. 2. [ I1 {(s/2 ) + 1} + I2 {(s/2 ) – 1}] ⒝
For 10% slope → s =0.1
∴ ⒜ → (- 0.95 I1 + 1.05 I2)
⒝ → ( 1.05 I1 - 0.95 I2)
Dividing across by 1.05, inputs become
(- 0.905 I1 + I2) and (I1 - 0.905 I2)
The phase comparator will now act like an amplitude comparator and
the characteristic is as given before.

38. FUSES
DEFINITION
1) Fuse → complete device including fuse-holder and fuse-link
2) Fuse link → actual fuse element which ‘blows’
Fuse vs. Mechanical Interrupter
• Fuse can interrupt very large current in shorter time, even before
peak is reached.
• Fuse has to be replaced whenever it acts.
• Fuse has poor protection against small current due to fusing factor >
1.25
• Fuses are cheaper than C.B. of similar rating and breaking capacity.
• Maintenance costs are lower for a fuse.
• Cost of replacements must be factored in when decision is being
made about a fuse.

40. Exploded View of Cartridge
Fuse

41. They consist of one of more parallel connected elements
which are made of materials of low resistivity.
Fuse materials should possess the following properties:
a. Low specific heat
b. High thermal conductivity
c. Low melting and vapourisation temperatures
d. Low latent heats

42. FUSES
Techniques of time delay on a selection of types of fuse element

43. Semi. Enclosed or Rewirable Fuse

44. Expulsion Fuse

45. Current-time relationships for a.c. and d.c. circuits

46. Current-time relationships at voltages close to or above
voltage rating

48. Peak arc voltage is dependant upon the number of constrictions in an
element, because of the arcs of the series. This gives a minimum value of
peak arc voltage irrespective of the applied voltage up to a certain point.
When this point is exceeded the extra applied voltage can force the arcing
to persist and produce burn-back and other effects which may increase
with each incremental voltage, thus a causing larger peak arc voltage.
This is illustrated in the Arc voltage characteristic below.
It is therefore clear that a fuse of higher voltage rating should not be used
to replace a blown fuse of lower voltage rating unless due cognisance is
taken of the fact that its peak arc voltage will be greater. Peak arc voltage
must not exceed the dielectric withstand of the system in which the fuse is
placed.
Fuses for 11kV use are frequently designed to produce low arc voltages, in
order that they may also be used on 6.6kV systems. It should not be
assumed that this is the case without first consulting the manufacturer or
his literature on this point.

50. Time/current characteristic and factors affecting it
When a conductor of resistance R Ω is being heated by the passage of
current i through it for a time dt, the quantity of heat liberated in the
conductor is i2Rdt. In other words, i2dt Joules are liberated for every ohm
of conductor. If the current is varying over a period then ∫i2dt Joules will
be liberated in the conductor for every ohm of resistance. This integral is
called the ‘Joule integral’ and is usually abbreviated to I2t. It is a most
convenient way of estimating the heating effect on a protected circuit due
to a very short pulse of heavy current.
If a fuse ink is blown on a very high prospective current, there is no time
for the heat to be lost into the surroundings and it is all used in heating
the element to the melting point at its narrowest constriction. In this
short time region, therefore, the I2t required to melt the element is
constant and independent of current. This is called the pre-arcing I2t.
Consider the section of fuse element shown below
The heat produced in time dt is

51. This quantity of heat will raise the temperature by θ if no
heat is lost to the surroundings
But m= DA l(where D = density)
Integrating both sides gives:
This means that if 2 = melting point of the metal of the element and
1 = 20˚C then, if the fuse begins its arcing time immediately the
element first melts pre-arcing∫i2 dt = K A2 (where K is a constant for the
metal, directly calculable from the values) i.e. pre-arcing I2t is
proportional to the square of the cross-sectional area of the section
melted.

52. Examples are given the following table for typical metals used as fuse
elements:
Metal Pre-arcing I2t
Silver 6.6 x 104 A2 amp2 sec A= cross-sectional area of
conductor at narrowest point
(in mm2)
Copper 9 x 104 A2 amp2 sec

53. Typical I2t characteristics

54. PROTECTION OF MOTORS
Point to Note:
The time/current characteristic of fuse link ( c ) must lie to the right of
the point s on the motor characteristic by an adequate amount.

55. Discrimination between H.V. and L.V. in two alternative circuits

56. IDMT
Non-Directional Overcurrent and
Earth Fault Protection
- Principles of Overcurrent protection
- Definition of terms used
- Types of Overcurrent Relay
- Calculation of Settings for Relay co-ordination

57. PURPOSE OF PROTECTION
- Detect abnormal conditions
- Isolate faulty part of system
- Fast operation to minimise damage and danger
- Discrimination – isolates only faulty section
- Dependable
- Secure
- Cost – balanced against cost of potential hazards

58. CO-ORDINATION
Co-ordinate protection so that relay nearest to fault
operates first – minimises amount of system
disconnection.

59. Fuses - simple
- can be very fast
- limit fault energy
- require co-ordination
- limited sensitivity for earth faults
- single phasing
- fixed characteristic
- need to replace

60. -Cut-Off - Characteristic

61. FUSE CO-ORDINATION
RATED CURRENT RATED CURRENT
= IFA = IFB
Simple “Rule of Thumb” for grading choose IFA ≏ 2* IFB
Preferably Need to Consider
Total I2t - MINOR FUSE
Pre-Arcing I2t - MAJOR FUSE
USE MANUFACTURERS’ “BULLRUSH” DIAGRAMS

62. CO-ORDINATION OF OVERCURRENT
RELAYS
- By means of relay current setting using instantaneous
overcurrent relays.
- By means of relay operating time using definite time
delay overcurrent relays.

63. INSTANTANEOUS OVERCURRENT
- Relies on difference in fault level at different system
locations
- Current setting chosen so that only relay nearest to fault
operates.
PROBLEM
Fault levels at F1 and F2 are essentially the same, therefore
cannot discriminate between A & B.

64. ADVANTAGE : Fast
A B C
F1 F2 F3
CURRENT
F1
F2
F3
DISTANCE

65. DEFINITE (INDEPENDENT)
TIME OVERCURRENT
- Relay operating time is independent of current magnitude
- Relay furthest away from source has shortest operating
time
PROBLEM
Longest time delay is at the source where the fault level will be
the highest.

66. DISCRIMINATION BY TIME AND
CURRENT (DEPENDENT TIME)
- Operating time depends on fault current level
- Can get faster clearance times than using definite
time delay relays
- Discrimination easier to achieve than with
instantaneous relays

67. INVERSE DEFINITE MINIMUM TIME
(IDMT) OVERCURRENT RELAYS
Operating time is Inversely Proportional to the current level

69. Static
Modular
Overcurrent
Relay-IDMT
Curves

70. CHOICE OF CURRENT SETTING
- Must allow for resetting of upstream relay when fault is
cleared by downstream protection
- If upstream relay does not fully reset, it’s operating time
of a subsequent fault will be reduced and discrimination
may be lost
- If R = Relay resetting ratio (Drop off/Pick up)
Is = IF L/R
To allow for resetting with full load current flowing through the
relay
R = 95% for MCGG and
R = 90% for CDG relay

71. TRANSIENT OVERREACH
Concerns Relay Response to offset Waveforms (DC Transient)

72. DEFINITION
I1 - I2
*100
I2
I1 = Steady state RMS pick up Current
I2 = Fully offset RMS pick up Current
I2
I1
D.C.
I1 = rms value which could just pick up relay
I2 = rms value of fully offset current which would just pick up relay

73. GRADING MARGIN
- Circuit Breaker Fault Interrupting Time
- Relay Overshoot Time (not actual time during which
forward operation continues, but time which would
be required to achieve same advance if relay still
energised).
- Relay Timing and CT Errors
- Safety Margin

74. Overshoot - t1 t3
Normal travel system unchanged - t1 t2

75. GRADING MARGIN
Er = relay timing error
Ect = CT ratio error allowance
t = operating time of relay nearest to fault
tcb = CB interrupting time
to = Relay Overshoot time
ts = Safety Margin

76. For example, grading between CDG relays
Er = 7.5%
Ect = 10.0%
tcb = 0.1 sec
to = 0.05 sec
ts = 0.1 sec
t’ = 0.25t + 0.25