1. Prof. Mahmoud El Bahy
Professor of High Voltage Engineereing
LECTUERE 1
E 1435 POWER SYSTEM PROTECTION
BENHA UNIVERSITY
FACULTY OF ENGINEERING AT BENHA
DEPARTMENT OF ELECTRICAL ENGINEERING
3. 1. INTRODUCTION
The power system is divided into protection
zones defined by the equipment and the
available circuit breakers. Six categories of
protection zones are possible in each power
system:
(1) generators and generator–transformer
units, (2) transformers,
(3) buses,
(4) lines (transmission, subtransmission, and
distribution),
(5) utilization equipment (motors, static loads,
or other), and
(6) capacitor or reactor banks (when separately
protected).
4. 2- Zones and General Principles of Protection
Figure 1 (a) : Primary relay protection zones.
6. In Figure 1, the protection zones of a simple power system are
shown. Each zone protects a single element of the power system.
The protection zones overlap around circuit breakers. The
purpose is to protect all sections of the power system. Typical
zones of protection with transmission lines, buses, and
transformers, each reside in its own zone. Closed zones in which
all power apparatus entering the zone is monitored.
Protection Zones
General Principles of Protection
Primary relays are relays within a given protection zone
that should operate for prescribed abnormalities within
that zone. In Figure 1 , for example, consider a fault at
line JK. For this condition, relays supervising breakers J
and K should trip before any others and these relays are
Protection zones (primary protection zones) are regions
of primary sensitivity. Figure 1 shows a small segment of
a power system with protection zones enclosed by
dashed lines.
7. Back-up protection is provided to ensure that the faulted element
is disconnected even if the primary protection fails to isolate the
faulted element. Back-up protection can be provided locally or
from a remote location.
Local backup relays are an alternate set of relays in a
primary protection zone that operate under prescribed
conditions in that protection zone. Often such local backup
relays are a duplicate set of primary relays set to operate
independently for the same conditions as the primary set.
This constitutes an effective safeguard against relay
failures.
Remote back-up protection is provided by equipment that is
physically located at substations away from the location where
equipment for primary protection is located. For example,
suppose a fault at line JK of Figure 1 cannot be cleared by
breaker J due to relay or breaker J malfunction. Backup
relays at locations I and M should be set to operate for the
fault at line JK, but only after a suitable delay that would
8. Reliability of a protective system is defined as the
probability that the system will function correctly when
required to act.
Sensitivity in protective systems is the ability of the
system to identify an abnormal condition that exceeds a
nominal "pickup"
Selectivity in a protective system refers to the overall
design of protective strategy wherein only those
protective devices closest to a fault will operate to remove
the faulted component.
The security property is defined in terms of regions of a power
system called zones of protection. A relay will be considered
secure if it responds only to faults within its zone of protection.
Undesired tripping (false tripping) results when a
relay trips unnecessarily for a fault outside its
protection zone or when there is no fault at all. This
can occur when the protective system is set with too
9. Basic parts of protective relaying mechanism:
(i) First part is the primary winding of a current transformer
(C.T.) which is connected in series with the line to be protected.
(ii) Second part consists of secondary winding of C.T. and the
relay operating coil. The relay coil makes the trip circuit when it
is energized sufficiently high to drag the arm to close the contacts
of the trip circuit.
(iii)Third part is the tripping circuit which may be either a.c. or
d.c. It consists of a source of supply, the trip coil of the circuit
breaker and the relay stationary contacts.
12. 3- Classification and Function of
Relays
A protection relay is a device that senses any
change in the signal it is receiving , usually from
a current and or voltage, if the magnitude of the
incoming signal is outside a pre-set value the
relay will carry out a specific operation, generally
to close or open electrical contacts to initiate
some further operation, for example the tripping
of a circuit breaker , figure 2.
13. A-Classification based on the construction and
principle of operation:
1. Electromagnetic relays. They are activated by
A.C. or D.C. quantities.
2. Electro-thermal relays. Thermal protection
using Bi-metallic strip.
3. Physico-electric relays. Change in the physical
parameters (Buchholz relay).
4. Static relays. use solid state devices for their
operation.
5. Microprocessor based relays. Use VLSI
technology.
Classification of Relays:
14. 1. Over current relays. Operate when the
activating quantity (current) rises above a
specified value.
2. Under voltage relay. Operates when the
activating quantity (voltage) falls below a
specified value.
3. Distance relays. Its operation depends upon
ratio.
4. Differential relays. Its operation depends upon
comparison of two or more electrical quantities.
5. Directional relays.
B- Classification based on its application:
15. 1. Instantaneous relays. Operation takes place
after a small interval of time that is
negligible.
2. Definite time relays. Its operation is
independent of magnitude of activating
quantity.
3. Inverse time relays. Their time of operation
is inversely proportional to the magnitude of
the activating quantity.
C- Classification based on time of operation:
18. Electromechanical relays represent a mature technology for
protective devices that have been widely used for many
years and are still applied for many purposes. These devices
have been proven to be reliable and are often favored by
protection engineers for many applications because of their
reliable performance and low cost. The characteristics of
electromechanical relays have been exhaustively treated in
the literature.
The relays are constructed with electrical, magnetic and
mechanical components and have an operating coil and
various contacts. Their construction characteristics can be
classified in two groups, Attraction Relays and Induction
Relays.
4- Electromechanical Relays
19. A- Attraction Relays
Attraction relays operate by the movement of a piece of
metal when it is attracted by the magnetic field produced by
a coil. The attracted relay types are armature and solenoid
relays, figures 3 a and b.
It can be shown that the force of attraction is equal to
(K1I2 - K2 ), where K1 depends upon the number of turns on
the operating coil, the air gap, the effective area and the
reluctance of the magnetic circuit, K2 is the restraining force
produced by a spring. When the relay is balanced, the
resultant force is zero and (K1I2 = K2 ), so that I2 = K2 / K1 =
constant, I is called pick up current (I pick up ). Attraction
relay has no time delay and for that reason it used when
( a ) Armature type relay
20. For current above the threshold I pick up the force
developed by the solenoid plunger overcomes the force
of gravity and closes the open contacts. The solenoid
relay is often referred to as an "instantaneous relay,"
The speed of this type of relay actually depends on
the magnitude of current flowing in the solenoid, and
if the current is large the relay will trip in about one
cycle.
23. Figure 3: ( C ) Characteristic of Solenoid type relay
24. Induction relays can be grouped into three classes;
shaded pole type, wattmetric and cup type relays,
figures 4 to 6. Figure 4 illustrates one of the induction
disk protective relaying (shaded pole relay). The disk can
be caused to rotate due to eddy currents that flow in the
disk, the currents being induced due to the fields
established by the poles. There are many ingenious
forms of induction disk relays. This one measures only
the current, but the shape of the relay time-current
characteristic can be changed to represent the various
generic types described in Figure 3. The time required to
trip for a given current depends on the angle of rotation
required to cause the movable contact to reach the fixed
contact. This angle, and hence the time to trip, is
adjustable by the "time lever" or dial setting, whereby the
fixed contact can be adjusted to a desired angular
displacement. This simple feature makes the relay very
B- Induction Relays
30. Induction relay consists of an electromagnetic
system which operates on a moving conductor,
generally in the form of a disc or cup, and functions
through the interaction of electromagnetic fluxes with
the eddy currents that are induced in the rotor by
these fluxes. These two fluxes which are mutually
displaced both in angle and in position, produce a
torque that can be expressed by
31. Typical overcurrent relay characteristics for a family of
similar relays of the same manufacturer are shown in
Figure , which distinguishes qualitatively between
characteristic shapes from definite time (bottom curve)
to extremely inverse time (top curve). Most relay
manufacturers offer these various characteristics in
electromechanical relays of the same basic type of
induction disk devices.
The induction disk relay can be analyzed by summing
the torques acting on the disk. The current flowing in
the poles develops a flux that creates eddy currents in
the induction disk. These currents interact with the flux
to produce torque that tends to rotate the disk. The
spring creates a retarding torque. A damping torque is
also produced that is proportional to the angular
velocity of rotation. We can summarize these torques
as follows:
32. The driving torque is proportional to the square of the
current in the current coil, the spring torque is a
constant retarding torque, and the damping torque is
proportional to the angular velocity. Therefore, we
may write as follows :
where appropriate constants of proportionality
have been introduced. We can determine the first
constant by means of a simple experiment. If the
disk is at rest, the right-hand side is zero. If we
slowly increase the current until the disk begins to
rotate, this establishes the threshold value of
current, which is usually called the "pickup"
34. It is reasonable to ignore the initial acceleration of the
disk, since the disk is very light and accelerated quickly
to its final constant velocity. If this simplification is
introduced, we approximate (3.5) as
for any current greater than pickup. As long as this
current continues to flow, the disk rotates at constant
velocity until the contacts close. If we designate the
angle of travel required to make these contacts as Op
we can find the time required for pickup. This time is
given by
35. where t p is the time to pickup. Note that the coefficient
in the numerator on the right-hand side has the
dimensions of seconds and is therefore recognized as
the time constant TI. This time constant is a relay
design parameter and will have different values
depending on the shape of relay characteristic curve
that is desired.
The foregoing ignores the saturation of the magnetic
circuit. Large currents, corresponding to large values of
M, cause the electromagnet to saturate. This causes the
flux to reach a
limiting value, which produces a constant operating
time, which we designate as T2 • However, large values
of M causes tp calculated above to approach zero.
Therefore, the effect of saturation is to add a constant
T2 . Last equation also fails to account for the fact that
some relay designs require different exponents of the
variable M. We can account for these additional
36. where we have added a second time constant to
account for saturation and have changed the exponent
on M to a variable p that can be changed according to
the relay design. Commercially
available induction disk relays have values of the
exponent p that vary over a rather wide range. This
flexibility, in addition to being able to select the two -
time constants in makes it possible to develop relays
with many different characteristics.
37. Figure gives a semi logarithmic plot of TC characteristics
for an inverse relay to illustrate the many choices of time
dial settings that are usually available on devices of this
type . As noted in Figure , the various time dial or lever
settings provide a means of adjusting the angle traveled
by the rotating contact in order to reach the fixed contact.
The various curves shown in Figure correspond to
distinct settings of the time dial or lever setting. The relay
used for the illustration has an inverse characteristic that
corresponds to one of the steeper curve s shown in Figure
. The adjective "inverse" is a relative term . Thus we see
inverse, moderately inverse . very inverse, and many other
names used to distinguish the various characteristics.
38. Typical overcurrent relay characteristics for a family of
similar relays of the same manufacturer are shown in
Figure, which distinguishes qualitatively between
characteristic shapes from definite time (bottom curve)
to extremely inverse time (top curve). Most relay
manufacturers offer these various characteristics in
electromechanical relays of the same basic
type of induction disk devices.
39. Figure 5 (a ) Schematic diagram for wattmetric type relay
