Transcript Feeder Protection Presentation

Feeder Protection
What is a Feeder?
• Overhead lines or cables which are used to
distribute the load to the customers. They
interconnect the distribution substations
• This is an electrical supply line, either
overhead or underground, which runs from
the substation, through various paths, ending
with the transformers. It is a distribution
circuit, usually less than 69,000 volts, which
carries power from the substation. with the
loads.
Why Protection Is Important?
• The modern age has come to depend
heavily upon continuous and reliable
availability 0f electricity and a high quality of
electricity too. Computer and
telecommunication networks, railway
networks, banking and continuous power
industries are a few applications that just
cannot function without highly reliable power
source.
• No power system cannot be designed in
such a way that they would never fail. So,
protection is required for proper working.
Basic Requirements of
Protection
• A protection apparatus has three main functions:
1. Safeguard the entire system to maintain continuity of
supply
2. Minimize damage and repair costs where it senses
fault
3. Ensure safety of personnel
• Protection must be reliable which means it must
be:
1. Dependable: It must trip when called upon to do so.
2. Secure: It must not trip when it is not supposed to.
Basic Requirements of
Protection
• These requirements are necessary for early
detection and localization of faults and for prompt
removal of faulty equipment from service.
• Selectivity: To detect and isolate the faulty item only.
• Stability: To leave all healthy circuits intact to ensure
continuity or supply.
• Sensitivity: To detect even the smallest fault, current
or system abnormalities and operate correctly at its
setting before the fault causes irreparable damage.
• Speed: To operate speedily when it is called upon to
do so, thereby minimizing damage to the
surroundings and ensuring safety to personnel.
What Is Fault?
• A fault is defined as defect in electrical
systems due to which current is directed
away from its intended path.
• It is not practical to design and build electrical
equipment or networks to eliminate the
possibility of failure in service. It is therefore
an everyday fact that different types of faults
occur on electrical systems, however
infrequently, and at random locations.
Classification of faults
• Faults can be broadly classified into two
main areas
which have been designated as
• Active faults
• Passive faults
Active Faults
• The ‘active’ fault is when actual current
flows from one phase conductor to another
(phase-to-phase), or alternatively from one
phase conductor to earth.
• This type of fault can also be further
classified into two areas
• Solid Fault
• Incipient Fault
Solid Faults
• The solid fault occurs as a result of an immediate
complete breakdown of insulation as would
happen.
• In these circumstances the fault current would be
very high resulting in an electrical explosion.
• This type of fault must be cleared as quickly as
possible, otherwise there will be:
– Increased damage at fault location
– Danger of igniting combustible gas in hazardous
areas
– Increased probability of faults spreading to healthy
phases
Incipient Fault
• The incipient fault is a fault that starts as a
small thing and gets developed into
catastrophic failure.
• Some partial discharge in a void in the
insulation over an extended period can
burn away adjacent insulation, eventually
spreading further and developing into a
‘solid’ fault.
Passive Faults
• Passive faults are not real faults in the true
sense of the word, but are rather conditions
that are stressing the system beyond its
design capacity, so that ultimately active
faults will occur. Typical examples are:
• Overloading leading to over heating of
insulation
• Overvoltage
• Under frequency
• Power swings
Transient and Permanent Faults
• Transient faults are faults, which do not
damage the insulation permanently and allow
the circuit to be safely re-energized after a
short period.
• Transient faults occur mainly on outdoor
equipment where air is the main insulating
medium.
• Permanent faults, as the name implies, are
the result of permanent damage to the
insulation.
Symmetric and Asymmetric
Faults
• A symmetrical fault is a balanced fault
with the sinusoidal waves being equal
about their axes, and represents a steadystate condition.
• An asymmetrical fault displays a DC
offset, transient in nature and decaying to
the steady state of the symmetrical fault
after a period of time.
Basic Fault Clearing Mechanism
• The main requirement of line protection is
1. In the event of short circuit, the circuit
breaker near to fault should open and all
other circuit breakers remain in closed
position.
2. If the circuit near to fault fail to trip, back up
protection should be provided by the adjacent
circuit breaker.
3. The relay operating should be the smallest
possible in order to preserve system stability
without unnecessary tripping of circuits.
Types of protection
• The need to analyze protection schemes
has resulted in the development of
protection coordination programs.
Protection schemes can be divided into
two major groupings:
• Unit schemes
• Non-unit schemes
Unit Type Protection
• Unit type schemes protect a specific area of the
system, i.e., a transformer, transmission line,
generator or busbar.
• The most obvious example of unit protection schemes
is based on Kerchief’s current law – the sum of the
currents entering an area of the system must be zero.
Any deviation from this must indicate an abnormal
current path. In these schemes, the effects of any
disturbance or operating condition outside the area of
interest are totally ignored and the protection must be
designed to be stable above the maximum possible
fault current that could flow through the protected
area.
Non unit type protection
• The non-unit schemes, while also intended to
protect specific areas, have no fixed boundaries.
As well as protecting their own designated areas,
the protective zones can overlap into other areas.
While this can be very beneficial for backup
purposes, there can be a tendency for too great an
area to be isolated if a fault is detected by different
non unit schemes.
• The most simple of these schemes measures
current and incorporates an inverse time
characteristic into the protection operation to allow
protection nearer to the fault to operate first.
Non unit type protection
Non unit type protection
• The non unit type protection system
includes following schemes:
– Time graded over current protection
– Current graded over current protection
– Distance or Impedance Protection
Over current protection
• This is the simplest of the ways to protect
a line and therefore widely used.
• It owes its application from the fact that in
the event of fault the current would
increase to a value several times greater
than maximum load current.
• It has a limitation that it can be applied
only to simple and non costly equipments.
Earth fault protection
• The general practice is to employ a set of two
or three over current relays and a separate
over current relay for single line to ground
fault. Separate earth fault relay provided
makes earth fault protection faster and more
sensitive.
• Earth fault current is always less than phase
fault current in magnitude. Therefore, relay
connected for earth fault protection is
different from those for phase to phase fault
protection.
Earth fault protection
Time graded protection
• This is a scheme of over current protection is
one in which time discrimination is
incorporated. In other words, the time setting
of the relays is so graded that minimum
possible part of system is isolated in the
event of fault.
• We are to discuss the application of the time
graded protection on
– Radial feeder
– Parallel feeder
– Ring feeder
Protection of radial feeder
• The main characteristic of the radial feeder is that
power can flow in one direction only from
generator to supply end of the load line.
• In radial feeder number of feeders can be
connected in series and it is desired that smallest
part of the system should be off in the event of
fault.
• This is achieved by time graded protection.
• In this system time setting time setting of a relay is
so adjusted that farther the relay from the
generating system lesser the time of operation.
Drawbacks of time graded
protection on radial feeder
• The drawbacks of graded time lag over
current protection are given below:
– The continuity in the supply cannot be
maintained at the load end in the event of fault.
– Time lag is provided which is not desirable in on
short circuits.
– It is difficult to co-ordinate and requires changes
with the addition of load.
– It is not suitable for long distance transmission
lines where rapid fault clearance is necessary for
stability.
Protection of parallel feeder
• For important installations continuity of supply is a
matter of vital importance and at least two lines
are used and connected parallel so as to share
load.
• In the event of fault occurring the protecting device
will select the faulty feeder and isolate it while
other instantly assumes increased load.
• The simplest method of obtaining such protection
is providing time graded over relays with inverse
time characteristics at one end and reverse power
directional relay at the other end.
Protection of ring main feeder
• The ring main is a system of inter
connection between a series of power
stations by an alternate route. The
direction of power flow can be changes at
will.
IDMT Relay
• In time graded protections IDMT (Inverse
definite minimum time) relays are used.
• As the name implies, it is a relay monitoring
the current, and has inverse characteristics
with respect to the currents being monitored.
This relay is without doubt one of the most
popular relays used on medium- and lowvoltage systems for many years, and modern
digital relays’ characteristics are still mainly
based on the torque characteristic of this type
of relay.
IDMT relay
Block diagram of IDTM Relay
It can be seen that the operating time of an IDMTL relay is
inversely proportional to function of current, i.e. it has a
long operating time at low multiples of setting current and
a relatively short operating time at high multiples of setting
current.
Current graded protection
• It is an alternative to time graded protection and is
used when the impedance between two
substations is sufficient.
• It is based on the fact that short circuit current
along the length of protected length of the circuit
decreases with increase in distance between the
supply end and the fault point.
• If the relays are set to operate at a progressively
higher current towards the supply end of the line
then the drawback of the long time delays
occurring in the graded time lag system can be
partially overcome.
•
DISTANCE OR IMPEDANCE
PROTECTION
A distance relay, as its name implies, has the
ability to detect a fault within a pre-set distance
along a transmission line or power cable from its
location.
• BASIC PRINCIPLE
The basic principle of distance protection
involves the division of the voltage at the relaying
point by the measured current. The apparent
impedance so calculated is compared with the
reach point impedance. If the measured
impedance is less than the reach point
impedance, it is assumed that a fault exists on the
line between the relay and the reach point.
BASIC PRINCIPLE OPERATION OF IMPEDANCE
RELAY
BALANCED BEAM PRINCIPLE OF IMPEDANCE
RELAY
• The voltage is fed onto one coil to provide
restraining torque, whilst the current is fed to
the other coil to provide the operating torque.
Under healthy conditions, the voltage will be
high (i.e. at full-rated level), whilst the current
will be low thereby balancing the beam, and
restraining it so that the contacts remain
open. Under fault conditions, the voltage
collapses and the current increase
dramatically, causing the beam to unbalance
and close the contacts.
Three stepped distance
protection
• Zone 1
First step of distance protection is set to
reach up to 80 to 90% of the length of the line
section. This is instantaneous protection i.e.
there is no intentional delay .
• Zone 2
second zone is requires in order to provide
primary protection to remaining 10 to 20% of
the line and a cover up to 50% of the next
line section. The operating time of this zone
is delayed so as to be selective with zone 1.
Three stepped distance
protection
• Zone 3
The third zone is provided with an intention to
give full back up to adjoining line section. It
covers the line of the section, 100% of the
next line section and reaches farther into the
system. The motivation behind the extended
reach of this step is to provide full back up to
the next line section. Its operating time is
slightly more than that of zone 2.
Main or Unit Protection
Main or Unit Protection
The graded over current systems described earlier
do not meet the protection requirements of a
power system. The grading is not possible to be
achieved in long and thin networks and also it can
be noticed that grading of settings may lead to
longer tripping times closer to the sources, which
are not always desired. These problems have
given way to the concept of ‘unit protection’
where the circuits are divided into discrete
sections without reference to the other sections.
The power system is divided into discrete zones.
Each zone is provided with relays and circuit
breakers to allow for the detection and isolation of
its own internal faults.
Back-up Protection
• It is necessary to provide additional
protection to ensure isolation of the fault
when the main protection fails to function
correctly. This additional protection is referred
to as ‘back-up’ protection.
• The fault is outside the zones of the main
protection and can only be cleared by the
separate back-up protection. Back-up
protection must be time delayed to allow for
the selective isolation of the fault by the main
or unit protection.
Types of Main Protection
• Following types of main or unit protections
are used in feeder networks
– Differential protection
– Carrier current protection using phase
comparison
– Translay Y protection system
Methods of obtaining selectivity
• The most positive and effective method of
obtaining selectivity is the use of differential
protection. For less important installations,
selectivity may be obtained, at the expense of
speed of operation, with time-graded
protection.
• The principle of unit protection was initially
established by Merz and Price who were the
creators of the fundamental differential
protection scheme.
Differential protection
• Differential protection, as its name implies,
compares the currents entering and
leaving the protected zone and operates
when the differential between these
currents exceeds a pre-determined
magnitude. This type of protection can be
divided into two types, namely
– Balanced current
– Balanced voltage
Balanced current Protection
• The CTs are connected in series and the
secondary current circulates between them.
The relay is connected across the midpoint
thus the voltage across the relay is
theoretically nil, therefore no current through
the relay and hence no operation for any
faults outside the protected zone. Similarly
under normal conditions the currents, leaving
zone A and B are equal, making the relay to
be inactive by the current balance.
Differential protection using current balance
scheme (external fault conditions)
Differential protection and internal fault conditions
•
Balanced current
Protection
The current transformers are assumed
identical and are assumed to share the
burden equally between the two ends.
However, it is not always possible to have
identical CTs and to have the relay at a
location equidistant from the two end CTs. It
is a normal practice to add a resistor in series
with the relay to balance the unbalance
created by the unequal nature of burden
between the two end circuits. This resistor is
named as ‘stabilizing resistance’.
McColl circulating current protection for single phase
systems
Balanced voltage system
• As the name implies, it is necessary to create
a balanced voltage across the relays in end A
and end B under healthy and out-of-zone
fault conditions. In this arrangement, the CTs
are connected to oppose each other .
Voltages produced by the secondary currents
are equal and opposite; thus no currents flow
in the pilots or relays, hence stable on
through-fault conditions. Under internal fault
conditions relays will operate.
Balanced voltage system – external fault (stable)
Balanced voltage system, internal fault (operate)
Translay Y Protection
system
• The system can be employed for the
protection of single phase or 3-phase
feeders, transformer feeders and parallel
feeders against both earth and phase
faults.
• It works on the principle that current
entering one end of the feeder at any
instant equals the current leaving the
feeder.
Translay Y Protection system
Advantages of Translay system
• The capacitance currents do not effect the
operation much.
• Only two pilot wires needed.
• The current transformers of normal
designs are employed i.e. air core type
• The pilot resistance do not effect the
operation as the major part of power is
obtained from CTs for operation.
Carrier current protection using
phase comparison
• In this type of relay we exploit the phase shift
undergone by the current at the end by which
is nearest to the fault.
• The end which is far from the fault cannot
discern any changes in the phase of fault
current and the closer end sees a sharp,
almost 180˚ change in the phase current.
• Under normal conditions, load currents and
external fault currents can be arranged to be
exactly out of phase but in case of internal
faults the currents become in phase.
Time taken to clear faults
• With the inherently selective forms of protection,
apart from ensuring that the relays do not operate
incorrectly due to initial transients, no time delay is
necessary. Operating times for the protection,
excluding the breaker tripping/clearing time are
generally of the following order:
–
–
–
–
Machine differential – few cycles
Transformer differential – 10 cycles
Switchgear (busbar) differential – 4 cycles
Feeder differential – few cycles
• These operating times are practically independent
of the magnitude of fault current.
Advantages of unit protection
• Fast and selective
Unit protection is fast and selective. It will
only trip the faulty item of plant, thereby ensuring
the elimination of any network disruptions.
• No time constraints
Time constraints imposed by the supply
authorities do not become a major problem
anymore.
• Future expansion relatively easy
Any future expansion that may require
another in-feed point can be handled with relative
ease without any change to the existing protection