Transcript Switchgear & Protection

Specification of CTs:
CTs are defined in terms of rated burden, accuracy class and accuracy limit.
Saturated values of rated burden are:
2.5, 5, 7.5, 10, 15 & 30 VA
Saturated accuracy limit factors are:
5,10,15, 20 & 30
Two accuracy classes (5% & 10 %) are quoted 5P & 10P which give a
composite error at rated accuracy limit. The method of describing at CT is
as following.
15 VA class 5P20
It means the burden is 15 and will not have more than 5% error at 20 times its
rated current.
* Design requirements of CT are specified in terms of knee point voltage,
magnetizing current at knee point and secondary resistance. There are
known in general as "class x" CTs.
Application:
In specifying CT's, the variation of impedance over the range
of setting any relays should be taken into consideration .
Example:
The normal burden of an over current relay is VA at setting, where the
normal setting range of the relay is 50% to 200% of nominal current.
There for a 1 A relay set to 50% would have:
Current setting = 50*1 = 0.5 A
Voltage across the coil = = 6V
Relay impedance = = 12 Ω
At 200 %
Current setting = 200*1 = 2 A
Voltage across the coil = = 1.5V
Relay impedance = = 0.75 Ω
If the characteristic of the relay should be maintained up to 20 times the relay setting, then
the Vkp not less than 20 * 6 V=120 V for a 50% setting
Or 20 * 1.5V= 30 V for a 200% setting.
However, the relay operating at 20 times its setting will have saturated
magnetically and therefore the impedance will be reduced. Hence,
in the case of the lowest setting core must be taken when
specifying.
The impedance at setting, which means that Vkp = 60 would be
satisfactory because the relay would have saturated before times.
For an earth fault relay having minimum setting of 20% would have
voltage at setting of:
3VA
 15V
0 .2 A
Voltage transformers VTs
One requirement should be used with protection, which is that the
secondary voltage must be accurate representation of the primary
voltage in both magnitude and phase. To meet this requirement, they
are designed to operate at low flux densities so that Ie , ration and
phase angle errors are small. This larger than that of power transformer,
which increase the overall size of the unit. The nominal secondary
voltage is sometimes 110V (line voltage).
Accuracy
VTs usually have range of voltage from 80% to 120 % and range
of burden from 25 % to 100 %. In protection when voltage
suppressed, accuracy measurement may be important during
fault condition.
Protection:
The primary side of VTs are usually protected by HRC fuses and fuses
or miniature circuit breaker on the secondary side
Residual Connection:
During earth fault of any one of the three phases, it is not possible to derive a
voltage in the conventional manner. Therefore, the residual (broken delta)
connection as shown in figure (3.5) must be used. Under the three-phase
balanced conditions the three voltage sum to zero. If one voltage is absent
during fault condition, then the difference in voltages between the phases
will be delivered to the relay
Fig. 3.5 Broken delta connection of a voltage transformer
Capacitor VTs:
The cost of the electromagnetic VTs at voltages of 132 kV or more is very high.
Thus, the capacitor VTs is proposed to be the more economical equipment.
It is virtually a capacitance voltage divide with a tuning inductance and an
auxiliary transformer as shown in figure (3.6).
Fig. 3.6 Capacitor voltage Transformer transformer
Power system protection:
Text protective Relay by J.Lewis Blackburn.
General philosophies :
What is a relay?
(IEEE) define a relay as an electric device that is designed to interpret
input condition in a prescribed manner and after specified condition
are met to respond to cause contact operation .Relay are utilized in
all as pacts of activity, the home ,communication , industry…..etc.
A protective relay is defined as a relay whose function is to detect
defective line or apparatus or other power system condition of an
abnormal or dangerous nature and to initiate appropriate control
circuit condition. Fuse are also used in protection and define as an
over current protective device with in a circuit opening fusible part
that is heated and severed by the passage of the over current
thought it.
A primary objective of all power system is to maintain a very high level of
condition of service, and to minimize the outage times when intolerable
conditions occur. Loss of power, dip of voltage and over voltage will occur
due to consequences of natural events, physical accident, equipment failure
a disoperation by human error.
Protection is the science, skill, and art of applying and setting and / or fuses to
provide maximum sensitivity to fault and undesirable condition.
ypical power circuit breaker:
Protective relays provide the "brains" to same trouble ,but as low energy device
they are able to open and isolate the problem area of the power system .
CBs and varions types of circuit interrupters are used to provide the
"muscle" for fault isolation .
Thus protective relays and interrupting devices are "team" . protective relays
without CBs have no basic value except for alarm. On the other hand , CBs
without protective relays are only energized or de energized manually.
Different type of voltages of typical CBs are shown in fig (1-7 &1-8).
Typical relay & CB connection :
Usually protective relays are connected to power system through CT and/or
VT. The circuit can be represented by a typical "one-line'" ac schematic and
dc trip circuit schematic as shown in fig (1-9) . in normal operation and when
CB(52) is closed , it is contact closes to energize the CB trip coil 52T, which
function to open breaker main contact and de energize the connected
circuit. The relay contacts are not designed to interrupt the CB trip coil
current so an auxiliary relay is used to "seal in" or by pass the protective
relay. Then 52a will open to de energize the breaker coil.
Fig-1.9 Typical single –line ac connection of a protective relay with its de trip schematic
Basic objectives of system protection :
Protection does not mean prevention, but minimizing the duration of
the trouble, the five basic objectives are:
i) Reliability: assurance that the protection will perform correctly.
ii) Selectivity: maximum continuity of service with minimum system
disconnection.
iii) Speed of operation: minimum fault duration and consequent
equipment damage.
iv) Simplicity: minimum protective equipment and associated circuitry
to achieve the protection objectives.
v) Economics: maximum protection at minimum total cost.
Classification of Relays:
Classification can be done by different ways, such as by function,
input, performance characteristics an operating be divided into five
types:
i) Protective Relays:
Protective relays and fuses operate on the intolerable power system
conditions. They are applied to all parts of the power system;
generates, buses, TFs, TLs, distribution lines and feeds, motors,
loads, capacitors banks and reactors. Fuses are usually used for low
voltage level (480 V).
ii) Regulating Relays:
Regulating relays are associated with tap changer of TFs, on governor
of generating equipment to control the voltages level with varying
load (used during normal conditions).
iii)Reclosing, synchronism check, synchronizing relays:
Relays of this type are used in energizing or restoring lines to service
after an outage and in interconnecting pre-energizing parts of the
systems.
iv) Monitoring Relays:
Relays of this type are used in energizing or restoring lines to service
after an outage and in interconnecting pre-energizing parts of the
systems.
Ivv) Auxiliary Relay:
There are two categories: contact multiplication (repeat contactors)
and circuit isolation.
Other relay classification:
Protective relays classified by input are known as current, voltage, power, frequency
and temperature relays. Those classified by operating principles are
electromechanical, solid state …etc. those classified by performance are distance,
reactance, over current ….etc.
Induction Relays
Torque id produced by applying two alternating fields to a moveable disk, which are
displaced in space and time.
I
I
b
I
a
t
The applied torque would accelerate the disc to a speed limited only by friction and windage
control can be done by two ways:
1- By permanent magnet whose field passes through the disk and produces a breaking force ,
which control the time characteristic of the relay .
2- By control spring which produces a torque proportional to disc angular disc placement.
Which is an inverse time characteristic…
Over current protection:
The standard relay characteristic:
t = 3*(log M)-1 = (3)/(log M)
where, M: multiple of setting.
At twice setting current, operation time = 10 sec.
And at 10 * the setting current, operation time = 3 sec.
As shown in fig. (5.1).
Fig.5.1: Characteristic Curve. Inverse-Minimum Time Relay
The seven plug bridge positions would be marked:
1
50%
2
75%
3
100%
4
125%
5
150%
6
175%
7
200%
Grading Principles (GEC text):
Relay coordination can be achieved either by time or over current or a combination of both time
and over current in order to chive correct discrimination .
i – Discrimination by time :
This method uses time intervals to give the relay nearest to the fault to operate first. Figure (9.1)
Shows that the radial feeders have CBs at the in feed end of each section, where each
protection comprises a definite time delay over current relay in which the operation of current
sensitive element initiates the time delay element.
Figure (9.1)
Discrimination by current :
Fault currents varies with the fault positions due to the difference in impedances relays are set at a
tapered values such that only the relay nearest to the fault trips its CB, Fig (9.2).
iii- Discrimination by Both Time and Current:Because of the limitation imposed by two previous methods, a time / current characteristic has
evolved . The following example illustrates this method of coordination clearly .
Graphs:
Fig.5.3: Time/Current Curves. IDMT Relay
Fig.1.1: Distance protection, (a) Schematic. (b) Time/Distance graph.
Fig.1.2: Distance Relay Circuit (Moving Coil Relay)