2. BASIC COMPONENTS OF
PROTECTION
Protective current transformers
(CTs), Potential transformers (PTs)
Relays
Circuit breakers (CBs)
3.
4. TRANDUCERS (CTs and PTs)
They provide quantities on the secondary side, proportional to
those on the primary side. They may be designed for
instrumentation or protection. Design specifications of these two
categories of transformers are different even though the operating
principles are common
7. Instrument &
protection C.Ts
CTs for measuring purpose
saturate at low level to protect
connected instruments from
damage.
Protection CT must saturate at
higher level so that secondary
it must have linear relation
with large primary current in
under short circuit.
Problem : ratio error and
phase angle error
9. C.T ratios & burden
Common C.T ratios are:
50:5,
100:5,150:5,
200:5,250:5,300:5,
400:5,450:5,500:5,600:5,
800:5,900:5,1000:5,1200:5
Standard secondary
rating: 5 A or 1 A.
During fault the current
may reach up to 10 to 20
times the rated value.
Load on C.T & P.T is
called burden. It
describes the
impedance connected to
the secondary winding
but may specify the
volt-ampere delivered
to the load.
12. P.Ts
Normal rating for primary
= 12 kv
For secondary = 67 V (
line to neutral) or
110V ( line to line)
For HV and EHV line,
capacitance potential
divider circuits are used
The inductor L is tuned
with the Equivalent
capacitive reactance seen
into the supply system to
produce series resonance
and brings transformer
output voltage in phase
with input system voltage
14. A protective relay is defined as :
“ a relay whose function is to detect
defective lines or apparatus or other
power system conditions of an abnormal
or dangerous nature and to initiate
proper appropriate control circuit action .”
15. Protective relaying
The function of protective relaying is to
cause the prompt removal from service
of any element of a power system when
it suffers a short circuit, or when it
starts to operate in any abnormal
manner that might cause damage or
otherwise interfere with the effective
operation of the system. The relaying
equipment is aided in this task by
circuit breakers which are capable of
disconnecting the faulty element when
they are called upon to do so by the
relaying equipment
16. COMMON RELAY TERMS
Relay The term Relay generally refers to a device that
provides an electrical connection between two or more
points in response to the application of a control signal.
A device designed to respond to sudden
predetermined changes in one or more physical
quantities on the appearance of certain conditions in
the physical system controlling it.
Protective relay A relay designed to detect
abnormal conditions in electrical system, which may
be dangerous or undesirable and to subsequently
initiate suitable protective action.
Main protection : protection normally expected to
take initiative in case of a fault in the protected
zone.
17. Back-up protection The protection
provided to act as substitute for the main
protection in case of failure or inability of
the latter to perform its intended function.
Primary relay A relay which is energized
without the interposition of instrument
transformer or shunts.
Secondary relay A relay which is
energized by means of instrument
transformers.
Shunt relay A relay which is energized
by means of shunt inserted in the main
circuit.
18. Electromagnetic relay A relay, whose operation
depends on the force exerted by a magnetic
field.
Electromechanical relay An electrical relay
which includes a movement and some contacts.
Thermal relay A relay which operates due to the
action of heat generated by the passage of
electric current on some heat responsive
medium.
Static relay A relay having no movements or
contacts.
Pick up Value of current or voltage which is the
threshold above which the relay will close its
contacts.
Drop out value of current or voltage which is
the threshold value below which the relay will
open its contact or return to normal position or
stall.
20. Electromechanica
l relaying
Over the years, the following
types of basic
electromechanical relays
employing the above
principles have been
perfected.
•Plunger or attracted armature
•Balanced beam relay with
restraint.
•Induction disc
•Induction cup
.
22. Force of attraction is proportional to square of flux density in air gap. The
operating torque is proportional to square of the current.
F = (KI2 - K') Where F is the net force, K' is constant, I is
rms current of armature coil, and K' is the
restraining force.
KI2 = K'The threshold condition for relay
operation would therefore be reached
when
23. To reduce the effects of contact arcing and high "On-resistances", modern
contact tips are made of, or coated with, a variety of silver based alloys to
extend their life span as given in the following table.
Contact Tip
Material
Characteristics
Ag
(fine silver)
Electrical and thermal conductivity are the
highest of all metals, exhibits low contact
resistance, is inexpensive and widely used.
Contacts tarnish through sulphur influence.
AgCu
(silver copper)
"Hard silver", better wear resistance and
less tendency to weld, but slightly higher
contact resistance.
AgCdO
(silver cadmium oxide)
Very little tendency to weld, good wear
resistance and arc extinguishing
properties.
AgW
(silver tungsten)
Hardness and melting point are high, arc
resistance is excellent.
Not a precious metal.
High contact pressure is required.
Contact resistance is relatively high, and
resistance to corrosion is poor.
AgNi
(silver nickel)
Equals the electrical conductivity of silver,
excellent arc resistance.
AgPd
(silver palladium)
Low contact wear, greater hardness.
Expensive.
platinum, gold and
silver alloys
Excellent corrosion resistance, used
mainly for low-current circuits.
24. Attracted armature type:
Characteristics:
Instantaneous
operation : no time
delay is intestinally
given.
But time lag can be
obtained by using
inverse time
characteristics of a
fuse connected in
parallel to relay coil.
Pick up setting:
◦ By means of taps
provided in relay coil
◦ By adjusting the air
gap between the
armature and
magnetic circuit
26. Shaded Pole construction
Principle of operation is same as that
of a shaded pole motor i.e the flux
rotates in a direction from unshaded
to shaded portion .
27. 1. To produce net torque we
require two phase displaced
fluxes ( a single flux is not
sufficient)
2. Maximum torque is produced
if two fluxes are 90 degree
phase displaced
3. Net torque is steady since
torque equation has no time
varying quantity
T = K.φ1.φ2.sinθ
28. The inverse time
relay
Deflecting torque is directly
proportional to square of
current I2
Spring provides restraining
torque ( prop to angle of
travel δ) and permanent
magnet provides damping
torque ( prop to disc travel
speed d δ /dt )
29. Inverse Definite minimum time relay
At higher level of current, relay core saturates .
relay torque being proportional to square of flux density therefore
becomes constant and give a minimum definite time of operation at
higher fault level
30. Plug setting & PSM
Relay coil has several taps. A
shorting plug is placed in one of
the taps position. This is called
plug setting
The actual r.m.s current in a relay
coil is usually expressed as a
multiple of its plug setting
current (Is) which is known as
plug setting multipler (PSM).
Mathematically
PSM = actual relay current
/ (Plug setting x rated
relay current)
31. PSM & TMS
Determine the time of
operation of relay of
setting current 5 A ,2.2 s
IDMT having a plug setting
125%. TMS is 60%. It is
connected to a supply
circuit through a C.T of
400:5 ratio and fault
current is 4000 A.
32. solution
Since plug setting is 125% ad rated
current of relay is 5A. Relay will pick up at
a current = 5x1.25 = 6.25 A
Therefore Is = 6.25 A
Now fault current is 4000 A. Therefore,
secondary current of C,T = (4000)(5/400)
= 50 A
Hence, PSM = sec. current of C.T/ Is
= 50 / 6.25
= 8
33. Types of Inverse
time relays
There are three types of
inverse time relays:
standard inverse (IDMT)
t = 0.14 / ( I0.02 – 1 )
very inverse
t = 13.5 / ( I – 1 )
extremely inverse
t = 80 / (I2 – 1)
where “t” is the operating time
of relay and “I” is the PSM.
34.
35. Selection of PS and time of
operation
IL,max,<Ip<If,min
Circuit breaker operating time: 0.10
seconds (2 power cycles of 50Hz) for
OCB and 0.08 s for VCB.
Relay overshoot (also called over
travel) which accounts for the fact that
relay continue to operate due to its
inertia even after the fault has
interrupted. This may be high for
mechanical relays (about 0.1 second)
and negligible for numerical relays (20
ms)
Safety margin of 0.1 sec.
36. Static relay
IEC defines:
A relay in which the designed
response is developed by electronic,
magnetic or other components
without mechanical motion.
37. Advantages
Low C.T burden.
Absence of mechanical inertia and bouncing
contacts, high resistance to shock and
vibration.
Very fast response and long life.
Low maintenance owing to absence of moving
parts and bearing friction.
Quick reset owing to absence of over shoot.
Ease of providing amplification enables
greater sensitivity.
Low energy levels required in the measuring
circuits permit miniaturization of the relay
module.
38. Limitations of SSRs
Temperature sensitivity ………………
temperature compensation ccts, have
been developed (use of thermistors).
Aging ………………
Sensitivity to voltage spikes………………..can
be eliminated by filter and shielding.
Damage due to over
loading………………..eliminated by careful
design.
39. Basic construction of SSR
Basically protective relays
are analogue-binary
converters with measuring
functions. (See fig).
1. Measuring circuit: C.Ts or
V.Ts.
2. Measuring signal; output
of C.T or V.T.
3. Rectifiers: this converts
the measuring signal so
that it can be processed
by the subsequent stage.
4. measuring element:
comparator , that
compares the measuring
signal to a preset value.
40. 5. output element: amplifier used to amplify the weak
binary signal and transfers it to one or more controlled
elements.
6. output signal
7. controlled element; carry out the final switching action
and gives signal to trip coil of circuit breaker.
8. feed element: power supply of relay
9. power may be supplied either by :
an auxiliary voltage source or from the measuring
circuit i.e., C.T or V.T
41. Reed relay
Contact elements------- accurately positioned flexible
nickel-iron strips , called Reeds
Construction------------ reeds sealed into a closed
glass capsule in an inert environment.
Contact gap------------- about 2.5 mm or less
Contacts ---------------- special alloy gold welded to
inner ends of the reeds. Best & most expensive type
of reed contacts are mercury welded and are
completely bounce proof.
Applications:
They are an alternative to thyristors as tripping
relays and are simple and low cost.
45. 1- Matching transformer
2-Voltage limiter to cut voltage to a safer level to protect relay
internal circuitry from too high input voltage. A filter is also given
to filter out harmonics.
3-Multiplexer to select the signals to be measured. A sample of
each signal( voltage, current, power) is measured once in every
ms
5- The A/D measures the level of measured signal & transform it
to a numerical value. The A/D is usually an 8-bit ADC,
representing numerical values 0-255
4- Programmable attenuator between MUX and A/D to enable
accurate handling of both high and low current or voltage level.
