BEE3133 Electrical Power Systems

1
BEE3133 Electrical Power Systems

• Chapter 6 System Protection
• Rahmatul Hidayah Salimin

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Introduction

• System Protection the equipment use to detect
  and isolate the faulty section from the system
  automatically.

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Introduction

• Short circuit occur when equipment insulation
  fails due to system overvoltages caused by
• Lightning or switching surges
• Flashover line-line (caused by wind)
• Flashover to tree
• Insulation contamination by dirt/salt
• Mechanical failure
• Cable insulation failure
• Natural causes
• Tower/pole or conductor falls
• Objects fall on conductors

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Introduction

• Short circuit currents can be several orders of
  magnitude larger than normal operating currents
• If it is allowed to persist, may cause
• Damage to the equipment due to heavy currents,
  unbalanced current, or low voltage produces by
  the short circuit
• Fire and explosion effect equipment/people
• Disruption of service in the entire power system
  area

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Introduction

• Careful design, operation and maintenance of
  system protection can minimize the occurrence of
  short circuit but cannot eliminate them.

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Fault Currents and Voltages
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Function of System Protection

• Cause the prompt removal from service of any
  elements of power system when it suffers a short
  circuit, or when it start to operate in any
  abnormal manner that might cause damage or
  otherwise interfere with the effective operation
  of the rest of the system.
• Provide indication of the location and type of
  failure so that the data can be used to assist in
  expediting repair and analyzing the effectiveness
  of fault-prevention and mitigation features.

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Function of System Protection

• Why do we need system protection
• Detect fault
• Isolate faulted component
• Restore faulted component
• Aims
• Continued supply for rest of system
• Protect faulted part from damage

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Types of Protection

• A Fuses
• For LV Systems, Distribution Feeders and
  Transformers, VTs, Auxiliary Supplies
•  
• B - Over current and earth fault
• Widely used in All Power Systems
• Non-Directional
• Directional
•   

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Types of Protection

• C - Differential
• For Distribution Feeders, Busbars, Transformers,
  Generators etc
• High Impedance
• Low Impedance
• Restricted E/F
• Biased
• Pilot Wire

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Types of Protection

• D - Distance
• For Transmission and Sub-transmission Lines and
  Distribution Feeders,
• Also used as back-up protection for transformers
  and generators without signaling with signaling
  to provide unit protection e.g.
• Time-stepped distance protection
• Phase comparison for transmission lines
• Directional comparison for transmission lines
•  

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Types of Protection

• E - Miscellaneous
• Under and over voltage
• Under and over frequency
• A special relay for generators, transformers,
  motors etc.
• Control relays auto-reclose, tap change control,
  etc.
• Tripping and auxiliary relays

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Design Criteria/Characteristics
Economy
Simplicity
Speed
Reliability
Sensitivity
Selectivity
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Design Criteria/Characteristics

• Reliability
• Operate dependably and in healthy operating
  condition when fault conditions occur, even after
  remaining idle for months or years.
• Selectivity
• Clearly discriminate between normal and abnormal
  system condition to avoid unnecessary, false
  trips.
• Sensitivity
• Ability to distinguish the fault condition,
  although the different between fault and normal
  condition is small.

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Design Criteria/Characteristics

• Speed
• Fault at any point in the system must be detected
  and isolated rapidly to minimize fault duration
  and equipment damage. Any intentional time delays
  should be precise.
• Economy
• Provide maximum protection at minimum cost
• Simplicity
• Minimize protection equipment and circuitry

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Economic Factor

• Total cost should take account of
• Relays, schemes and associated panels and panel
  wiring
• Setting studies
• Commissioning
• CTs and VTs
• Maintenance and repairs to relays
• Damage repair if protection fails to operate
• Lost revenue if protection operates unnecessarily

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Economic Factor

• The cost of protection is equivalent to an
  insurance policy against damage to plant, and
  loss of supply and customer goodwill.
• Acceptable cost is based on a balance of
  economics and technical factors. Cost of
  protection should be balanced against the cost of
  potential hazards.
• There is an economic limit on what can be spent.
• MINIMUM COST Must ensure that all faulty
  equipment is isolated by protection.

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Relationship between reliability of supply, its
value and cost to the consumer
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System Protection Components

• Transducer / Instrument Transformer
• Relay
• Circuit Breaker

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System Protection Components

• Function
• Transducers/Instrument Transformers
• Provide low current and voltage, standardized
  levels suitable for the relays operation.
• Relays
• Discriminate between normal operating and fault
  conditions.
• When current exceed a specified value relay will
  be operated and cause the trip coil of CB to be
  energized/open their contact.
• Circuit Breakers
• Open the line

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System Protection Components
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System Protection Components
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System Protection Flow
voltage or current rise from normal condition
voltage/current is reduced to match with relay
rating
activate circuit breaker
circuit isolation
Relay
Transducer
Fault Occur
Circuit Breaker
Fault Clear
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Zones of Protection

• For fault anyway within the zone, the protection
  system responsible to isolate everything within
  the zone from the rest of the system.
• Isolation done by CB
• Must isolate only the faulty equipment or section

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Zones of Protection

• Zones are defined for
• Generators
• Transformers
• Buses
• Transmission and distribution lines
• Motors

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Zones of Protection
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Zones of Protection

• Characteristics
• Zones are overlapped.
• Circuit breakers are located in the overlap
  regions.
• For a fault anywhere in a zone, all circuit
  breakers in that zone open to isolate the fault.

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Overlapped of Protection

• No blind spot
• Neighboring zones are overlapped to avoid the
  possibility of unprotected areas
• Use overlapping CTs
• Isolation done by CB. Thus, it must be inserted
  in each overlap region to identify the boundary
  of protective zones.

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Overlapped of Protection

• Overlap accomplish by having 2 sets of instrument
  transformers and relays for each CB.
• Achieved by the arrangement of CT and CB.

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Primary Back-up Protection

• Primary protection is the protection provided by
  each zone to its elements.
• However, some component of a zone protection
  scheme fail to operate.
• Back-up protection is provided which take over
  only in the event of primary protection failure.

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Example

• Consider the power system shown below, with the
  generating source beyond buses 1, 3 and 4. What
  are the zones of protection in which the system
  should be divided? Which circuit breakers will
  open for faults at P1 and P2?

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Fault at P1 A, B, C Fault at P2 A, B, C,D, E
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Example

• If three circuits breakers are added at the tap
  point 2, how would the zones of protection be
  modified? Which circuit breakers will operate for
  fault at P1 and P2 under these conditions?

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Fault at P1 A, F Fault at P2 C,D,E,G
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Zone Discrimination

• A system as shown with relays and breakers
  marked. A single fault has resulted in the
  operation of breakers B1, B2, B3 and B4.Identify
  the location of the fault
• Answer
• Fault in the overlap zone at breaker B2 as shown

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Back-up Protection

• 1.Duplicate Primary
• Provide primary protection when the
  primary-relaying equipment is out of service for
  maintenance or repair
• Disconnect when primary relaying operates
  correctly
• Operate with sufficient time delay (coordination
  time delay) if primary not operate
• When short circuit occur, both primary and
  back-up start to operate, but if primary is
  operate, then the back-up will reset.

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Back-up Protection

• 2.Remote Back-up
• located outside boundary of Zone of Protection

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Example
C, D, E
A, B, F
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Example
C, D, E, F, G, H
A, B, I, J
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Transducers

• Also known as Instrument Transformer
• Use to reduce abnormal current voltage levels
  and transmit input signals to the relays of a
  protection system.
• Why do we need transducer
• The lower level input to the relays ensures that
  the physical hardware used to construct the
  relays will be small cheap
• The personnel who work with the relays will be
  working in a safe environment.

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Transducers

• Current and Voltage Transformers
• Correct connection of CTs and VTs to the
  protection is important directional, distance,
  phase comparison and differential protections.
• Earth CT and VT circuits at one point only

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VT and CT Schematic
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Voltage Transformers

• VT is considered to be sufficiently accurate.
• It is generally modeled as an ideal transformer.
• VT secondary connected to voltage-sensing device
  with infinite impedance.

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Voltage Transformers

• Types of VTs
• Electromagnetic VT
• Capacitive VT
• Busbar VTs
• Special consideration needed when used for line
  protection
• LV application(12 kV or lower)
• Industry standard transformer with a primary
  winding at a system voltage and secondary winding
  at 67 V(line-to-neutral) and 116 V(line-to-line).
  

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Voltage Transformers
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Voltage Transformers

• Voltage/Potential
• Transformer (VT/PT)

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Voltage Transformers
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Voltage Transformers
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Voltage Transformers

• HV and EHV
• Capacitor-coupled VT (CVT)
• C1 C2 are adjusted, so that a few kVs of
  voltage is obtains across C2
• Then, stepped down by T
• VTs must be fused or protected by MCB.

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Voltage Transformers
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Voltage Transformers

• VT ratios
• ratio of the high voltage/secondary voltage
• 11 21 2.51 41
• 51 201 401 601
• 801 1001 2001 3001
• 4001 6001 8001 10001
• 20001 30001 45001

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Current Transformers

• CT is an instrument transformer that is used to
  supply a reduced value of current to meters,
  protective relays, and other instruments.
• The primary winding consist of a single turn
  which is the power conductor itself.
• CT secondary is connected to a current-sensing
  device with zero impedance.

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Current Transformers

• CTs ratio(secondary current rating is 5A)
• 505 1005 1505 2005
• 2505 3005 4005 4505
• 5005 6005 8005 9005
• 10005 12005
• CTs also available with the secondary rating of 1A

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Current Transformers
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Current Transformers
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Reclosers and Fuses

• Automatic reclosers are commonly used for
  distribution circuit protection.
• Recloser self-controlled device for
  automatically interrupting and reclosing an AC
  circuit with preset sequence of openings and
  reclosures
• Have built-in control to clear temporary faults
  and restores service with momentary outages.
• Disadvantages
• increase hazard when circuit is physically
  contacted by people.
• Recloser should be locked out during live-line
  maintenance.

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Reclosers and Fuses

• An upstream fuse/relay has detected a fault
• Downstream system isolated by fuse or breaker
• Automatic re-closing after delay successful if
  fault not permanent

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Relays

• Discriminate between normal operating and fault
  conditions.
• Type of Relays
• Magnitude Relay
• Directional Relay
• Distance/Ratio Relay
• Differential Relay
• Pilot Relay

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Magnitude Relays

• Also called as Overcurrent Relay
• Response to the magnitude of input quantities ie.
  current.
• Energize CB trip coil when the fault current
  magnitude exceeds a predetermined value or trips
  when a current rises above a set point (pick-up
  current).
• If it is less than the set point value, the relay
  remains open, blocking the trip coil.
• Time-delay Overcurrent Relay also have the same
  operating method but with an intentional
  time-delay.

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Directional Relays

• Responds to fault only in one direction, either
  to the left or to the right of its location
• Operation depends upon the direction (lead or
  lag) of the fault current with respect to a
  reference voltage.
• The directional element of these relays checks
  the phase angle between the current and voltage
  of one phase, and allows the overcurrent unit to
  operate if this phase angle indicates current in
  the reverse direction.

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Ratio Relays

• Operate for certain relations between the
  magnitudes of voltage, current and the phase
  angle between them.
• Measures the distance between the relay location
  and the point of fault, in term of impedance,
  reactance and admittance.
• Respond to the ratio of two phasor quantities as
  example Voltage and Current (Z V/R)
• Also called impedance or distance relay

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Differential Relays

• Respond to the vector difference between two
  currents within the zone protection determined by
  the location of CTs.
• Not suitable for transmission-line protection
  because the terminals of a line are separated by
  too great a distance to interconnect the CT
  secondaries.
• For the protection of generators, transformers,
  buses,
• Most differential-relay applications are of the
  current-differential type.

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Differential Relays
Relay

• Fault occur at X
• Suppose that current flows through the primary
  circuit either to a load or to a short circuit
  located at X.
• If the two current transformers have the same
  ratio, and are properly connected, their
  secondary currents will merely circulate between
  the two CTs as shown by the arrows, and no
  current will flow through the differential relay.

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Differential Relays
Relay

• A flow on one side only, or even some current
  flowing out of one side while a larger current
  enters the other side, will cause a differential
  current.
• In other words, the differential-relay current
  will be proportional to the vector difference
  between the currents entering and leaving the
  protected circuit and, if the differential
  current exceeds the relays pickup value, the
  relay

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Differential Relays
Relay

• When a short circuit develop anywhere between the
  two CTs.
• If current flows to the short circuit from both
  sides as shown, the sum of the CT secondary
  currents will flow through the differential
  relay.
• It is not necessary that short-circuit current
  flow to the fault from both sides to cause
  secondary current to flow through the
  differential relay.

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Pilot Relays

• The term pilot means that between the ends of
  the transmission line there is an interconnecting
  channel of some sort over which information can
  be conveyed.
• Use communicated information from remote sites as
  input signals.

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Pilot Relays

• Transmitting fault signals from a remote zone
  boundary to relays at the terminals of a long TL
• Pilot relaying provides primary protection only
  back-up protection must be provided by
  supplementary relaying.
• Type wire pilot, carrier-current pilot and
  microwave pilot.

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Pilot Relays
ZA
ZB

• Station 1 consist of meter for reading voltage,
  current and power factor.
• Distance relay, tell the different between fault
  at A (middle) and B (end) by knowing the
  impedance characteristic per unit length of the
  line.

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Pilot Relays

• Could not possibly distinguish between fault B
  and C because impedance would be so small-
  Mistake in tripping CB for fault B or C
• Solution- indication from station B, when the
  phase angle of the current at S-B(with respect to
  current A) is different by approximately 180o
  from it value for fault in the line section AB.

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Pilot Relays
2
1
B
C
A
(with respect to current A) is different by
approximately 180o from it value for fault in the
line section
(with respect to current A) is not different in
degree from it value for fault in the line section