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Power systems Protection
course
Department of Electrical Power Energy Engineering Dr Audih alfaoury 1
Al-Balqa Applied University
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Overcurrent Protective Relays
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Part 6
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Overcurrent relays
Is the simplest and cheaper type of protection used for lines,
transformers, generators and motor.
Overcurrent relays Types:-
Based on operating time characteristics, normally defined by the
time vs. current curve (or T-I curve), there are three main types :
Instantaneous
Time-dependent ( Definite time or Inverse time)
Mixed ( Definite time + Inverse time)
1.Instantaneous Overcurrent Relays(50,50N) These relays operate without time delay, so they are called
instantaneous units (operating time= 0.1 second).
The simplest form of these relays are the magnetic attraction
types, Solid- State, digital and numerical over-current relays .
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Instantaneous Overcurrent Relays operate without time delay, ( 0.1s)
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The pickup current or threshold is adjustable **.
Pickup Current Setting
♣ Taps in the Relay Current Coil.
♣ Air-Gap Adjustment.
♣ Spring Adjustment.
Features of Electromechanical 50 Elements
♣ Suitable for AC and DC Systems.
♣ Operation Time Less Than 3 Cycles
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**smallest detectable value of current that will be operate the relay.
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Instantaneous Curve (50)
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Operation zone
of relay
Un operation
zone due 3
cycles
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2. Time Overcurrent Relays
Operate with a time delay.
Time delay is adjustable.
Pick up current is also adjustable.
There are five different types of time over-current relays
(depends on their time-current characteristic curves) :-
Definite time (DT) or (precise time).
Inverse-time :-
Moderately inverse
Inverse (Normal)
Very inverse
Extremely inverse
2.1 Definite-Time Overcurrent Relays (DT)
The definite-time relay operates with some delay this delay is
adjustable as well as the current threshold.
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Time over-current relays operate with a time delay
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2.2 Inverses-Time
This type of relay have an operating time depending
on the value of the current, generally with an inverse
characteristic,(increasing time decreasing current or
voice versa)
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The operation time of is smaller as the current gets larger
The operation time
of the relay is smaller
as the current gets
larger (small value
of time compared with
current value).
Also have two settings:
1- pick-up current and
2- curve level.
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In early electromechanical relays the curve is set by
means of a dial. Thus, the setting is called the “time dial
setting - TDS”.
The most common three
types of inverse curves
used are:
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o Standard inverse (SI)
o Very inverse (VI)
o Extremely inverse (EI).
IEEE does not specify coefficients in the standard curve equation. Thus, each manufacturer’s curve is
similar. But different IEC curves are standardized
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Typical Inverse-Time Overcurrent Relay Elements:
1. Induction disc over-current relay.
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2. Shaded-Pole Induction 51 Element
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Overcurrent Relay Setting
50 Elements: Pickup Setting
51 Elements Pickup setting
Time delay setting
definite time: time setting
inverse time: curve selection
Time-over-current relays (ANSI 51 relays) have two basic settings:
1- Pickup current and
2- Time delay settings.
The process for determining the time delay setting involves:
(1) Calculation of a time delay value in definite-time over-current
elements.
(2) Selection in inverse-time over-current elements of a time -
current curve from a family of curves.
Instantaneous over current (50) elements have only one setting: the pickup current.
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Note: The Selection of an Overcurrent Relay Time Curve According
to American Standard (ANSI)
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2.3 Mixed Curves Over-Current Relays
Mixed curves have all the advantages of the different types of over-
current relays. As the over current elements are built as separate
units, we may implement the over-current protection principles using:
a) a combination of instantaneous and definite-time elements .
b) a combination of instantaneous and inverse-time elements.
c) a combination of instantaneous, definite-time and inverse-time
elements.
d) a combination of definite-time and inverse-time elements(IDMT).
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Note: This type of characteristic is mainly used in digital relays.
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2.3.3 Mixed Curves (Inverse-Time +Definite -Time)
IDMT –Characteristics
The most commonly used type of relay is the inverse
definite with minimum time lag relay (IDMT) in which
inverse characteristic plus definite time characteristic
are used. The operating time is approximately
inversely proportional to the fault current near pickup
value and become substantially constant slightly above
the pick up value
of the relay.
The characteristic
is shown in Fig.
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Wattmetric and
shaded pole relays
of induction type can
be used to obtain
this characteristic.
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Standard Time and Inverse definite minimum time
(IDMT) Relay Characteristics
In American standard the time over-current relay are
five different types of characteristic curves.
− CO-6 Definite minimum,
− CO-7 Moderately inverse,
− CO-8 Inverse,
− CO-9 Very inverse,
− CO-11 Extremely inverse,
The time level settings are selected so that all relays
operate in 0.2 sec at 20 times the tap setting.
These time-current characteristics are compared in Figure in next
slides.
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Definite minimum
Moderately inverse
Inverse
Very inverse Extremely inverse
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CO-8 (normal inverse).
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CO-7 (moderate inverse).
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Typical time curves for standard British Standard (BS142) and
IEC Standard over-current relay (normal inverse).
Time Multiples Setting TMS= 0.1-1
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Typical IEC standard over-current relay(normal inverse).TMS=1.
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Curve Equation
The typical time curves for CO-8 American over-current
relay(normal inverse) characteristics can be approximated by the
following equation.
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The typical time curves for IEC standard overcurrent
relay(normal inverse) characteristics can be approximated by
the following equation.
Where :
TMS = Time multiplier setting
CTR = Current transformer ratio
PS = Plug setting
IF = Fault primary current
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(PS) and (TMS) in Over-current Relays
The working principle of an inverse time over-current
relay is shown in figure below.
Plug Setting (PS). The current feeds a coil is controlled with
multiple taps
Time dial setting: Is the time that allows the adjustment
between contacts and the operating time
. The generated magnetic field
makes the disc rotate with a
speed proportional to the
current
The braking magnet lessens
the rotating speed and acts as
an opposing force to the
rotation. Varying the
magnetization, different
tripping curves can be
achieved.
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Example:
Calculate the plug setting and time multiplier setting for an IDMTL
relay on the following network so that it will trip in 2.4 s .
(see Figure 1).The relay characteristic is shown in Figure 2.The C.T.
setting is 100/5 A and the fault current is 1000 A.
Figure 1
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Answer:
Fault current = 1000 A
CT ratio = 100/5 A
The current into relay under fault conditions,
Choose plug setting100% this is 5 A Therefore, current into relay as
a multiple of plug setting during fault is 50 / 5 = 10
We require the relay to operate after 2.4 s as soon as this much
current starts flowing in the circuit. Referring to characteristic curves
below, read time multiplier setting where 10 times plug setting
current and 2.4 s cross, which is about 0.8. Accordingly,
The relay settings is: PS=5 A (100%) and TMS=0.8.
Alternatively, if the current plug setting is chosen as 125% (6.25
A), the fault current through the relay will be 50/6.25 = 8 A. The
graph shows that eight times plug setting to operate in 2.4 s, the time
multiplier should be about 0.7.
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This technique is fine and give exactly required of the TM curve.
( )
51000 50A
100
Fop relay
II
CTR
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If the desired setting falls between the curves, and not easy to
estimate the intermediate setting accurately as the scales of the
graph are log/log. Then the following procedure is recommended.
From the scale below go to the multiple of plug setting current and
read the second value corresponding to the 1.0 time multiplier
curve. Then divide the desired time setting by this figure. This will
give the exact time multiplier setting:
Example if the second value at 10 times = 3 then for ( at 8 times it
is about 3.33 or 3.4) Desired setting = 2.4.
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3.4
8
2.43.4
Desired setting
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***Note the PS is given as 200% and we need to find the PSM
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Example 2:
Determine the time of operation of a relay of rating 5A, 2.2 sec
IDMTL and having a relay setting of 125%, TMS=0.6. It is
connected to a supply circuit through a C.T 400/5 ratio. The fault
current is 4000A.
Solution:
The operating current of the relay: 5×1.25=6.25A
Ip=IF=4000 A
If we select TMS=0.6 then the operation time is
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From the curve, the operating time for PSM=8 is 3.2 sec (TMS=1)
( This means that the current in secondary CT in case of fault and if we set the PSM
to 8 then the relay will operate at 6.25 A and 3.2 second )
0.6( )3.2 1.92sec.
1.0( )
TMS
TMS
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or From log to log curve (for
TMS=0.6)
Or from Figure scale figure (for TMS=1)
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1.92 sec≈
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Example 3:
If the rated current (pick up current) of a relay is 3A, and the time
dial setting(TDS) is 1.
(a) How long(time) does it take the relay to trip if the supply C.T is
rated at 400:5 A and the fault current is 480A? The type of the
OC relay is CO-8.
(b) Solve using the standard curve equation and compare the
results.
Solution:
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CO-8 Characteristics
b) Alternative method using curve
equation
Note:
TDS(time dial setting)=TD(time delay)
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Example 4:
For the relay R1 in the system shown determine the current tap
setting CTD. If the maximum three-phase fault current is 2400A and
the TDS=2.0.
Find the operating time if the relay type is CO-8 (inverse type).
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Solution:
Load current at the busbar1
Since the current tap setting (CTS) of CO-8 relay available are
4,5,6,7,8,10 and 12A
Hence we choose CTS=4 (this is relay operation current in case of
fault)
Fault current If=2400 A
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