Fundamentals of Distance Protection

1
Fundamentals of Distance Protection

• GE Multilin

2
Outline

• Transmission line introduction
• What is distance protection?
• Non-pilot and pilot schemes
• Redundancy considerations
• Security for dual-breaker terminals
• Out-of-step relaying
• Single-pole tripping
• Series-compensated lines

3
Transmission Lines

• A Vital Part of the Power System
• Provide path to transfer power between
  generation and load
• Operate at voltage levels from 69kV to 765kV
• Deregulated markets, economic, environmental
  requirements have pushed utilities to operate
  transmission lines close to their limits.

4
Transmission Lines

• Classification of line length depends on
• Source-to-line Impedance Ratio (SIR), and
• Nominal voltage
• Length considerations
• Short Lines SIR gt 4
• Medium Lines 0.5 lt SIR lt 4
• Long Lines SIR lt 0.5

5
Typical Protection SchemesShort Lines

• Current differential
• Phase comparison
• Permissive Overreach Transfer Trip (POTT)
• Directional Comparison Blocking (DCB)

6
Typical Protection SchemesMedium Lines

• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach Transfer Trip (PUTT)
• Permissive Overreach Transfer Trip (POTT)
• Unblocking
• Step Distance
• Step or coordinated overcurrent
• Inverse time overcurrent
• Current Differential

7
Typical Protection SchemesLong Lines

• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach Transfer Trip (PUTT)
• Permissive Overreach Transfer Trip (POTT)
• Unblocking
• Step Distance
• Step or coordinated overcurrent
• Current Differential

8
What is distance protection?

• For internal faults
• IZ V and V approximately in phase (mho)
• IZ V and IZ approximately in phase (reactance)

9
What is distance protection?

• For external faults
• IZ V and V approximately out of phase (mho)
• IZ V and IZ approximately out of phase
  (reactance)

10
What is distance protection?
IntendedREACH point
Z
RELAY
11
Source Impedance Ratio, Accuracy Speed
Relay
Line
System
Consider SIR 0.1
12
Source Impedance Ratio, Accuracy Speed
Relay
System
Line
Voltage at the relay
Consider SIR 30
13
Challenges in relay design

• Transients
• High frequency
• DC offset in currents
• CVT transients in voltages

14
Challenges in relay design

• Transients
• High frequency
• DC offset in currents
• CVT transients in voltages

15
Challenges in relay design
Sorry Future (unknown)

• In-phase internal fault
• Out-of-phase external fault

16
Transient Overreach

• Fault current generally contains dc offset in
  addition to ac power frequency component
• Ratio of dc to ac component of current depends
  on instant in the cycle at which fault occurred
• Rate of decay of dc offset depends on system X/R

17
Zone 1 and CVT Transients

• Capacitive Voltage Transformers (CVTs) create
  certain problems for fast distance relays applied
  to systems with high Source Impedance Ratios
  (SIRs)
• CVT-induced transient voltage components may
  assume large magnitudes (up to 30-40) and last
  for a comparatively long time (up to about 2
  cycles)
• 60Hz voltage for faults at the relay reach point
  may be as low as 3 for a SIR of 30
• the signal may be buried under noise

18
Zone 1 and CVT Transients

• CVT transients can cause distance relays to
  overreach. Generally, transient overreach may be
  caused by
• overestimation of the current (the magnitude of
  the current as measured is larger than its actual
  value, and consequently, the fault appears closer
  than it is actually located),
• underestimation of the voltage (the magnitude of
  the voltage as measured is lower than its actual
  value)
• combination of the above

19
Distance Element Fundamentals
XL
R
XC
20
Impedance locus may pass below the origin of the
Z-plane - this would call for a time delay to
obtain stability
21
CVT Transient Overreach Solutions

• apply delay (fixed or adaptable)
• reduce the reach
• adaptive techniques and better filtering
  algorithms

22
CVT Transients Adaptive Solution

• Optimize signal filtering
• currents - max 3 error due to the dc component
• voltages - max 0.6 error due to CVT transients
• Adaptive double-reach approach
• filtering alone ensures maximum transient
  overreach at the level of 1 (for SIRs up to 5)
  and 20 (for SIRs up to 30)
• to reduce the transient overreach even further an
  adaptive double-reach zone 1 has been implemented

23
CVT Transients Adaptive Solution

• The outer zone 1
• is fixed at the actual reach
• applies certain security delay to cope with CVT
  transients

• The inner zone 1
• has its reach dynamically controlled by the
  voltage magnitude
• is instantaneous

24
Desirable Distance Relay Attributes

• Filters
• Prefiltering of currents to remove dc decaying
  transients
• Limit maximum transient overshoot (below 2)
• Prefiltering of voltages to remove low frequency
  transients caused by CVTs
• Limit transient overreach to less than 5 for an
  SIR of 30
• Accurate and fast frequency tracking algorithm
• Adaptive reach control for faults at reach points

25
Distance Relay Operating Times
26
Distance Relay Operating Times
35ms
25ms
30ms
20ms
15ms
27
Distance Relay Operating Times
SLG faults
LL faults
3P faults
28
Actual maximum reach curves
29
Maximum Torque Angle

• Angle at which mho element has maximum reach
• Characteristics with smaller MTA will
  accommodate larger amount of arc resistance

30
Mho Characteristics
Traditional
Directional angle slammed
Directional angle lowered and slammed
Both MHO and directional angles slammed (lens)
31
Load Swings
XL
LOOKING INTO LINE normally considered forward
Reach
Load Trajectory
Operate area
No Operate area
Typical load characteristic impedance
R
32
Load Swings
Lenticular Characteristic
Load swing
33
Load Encroachment Characteristic
The load encroachment element responds to
positive sequence voltage and current and can be
used to block phase distance and phase
overcurrent elements.
34
Blinders

• Blinders limit the operation of distance relays
  (quad or mho) to a narrow region that parallels
  and encompasses the protected line
• Applied to long transmission lines, where mho
  settings are large enough to pick up on maximum
  load or minor system swings

35
Quadrilateral Characteristics
36
Quadrilateral Characteristics
Ground Resistance (Conductor falls on ground)
R
Resultant impedance outside of the mho operating
region
XL
37
Distance Characteristics - Summary
Mho
Quadrilateral
Lenticular
JX
R
Better coverage for ground faults due to
resistance added to return path
Standard for phase elements
Used for phase elements with long heavily loaded
lines heavily loaded
38
Distance Element Polarization

• The following polarization quantities are
  commonly used in distance relays for determining
  directionality
• Self-polarized
• Memory voltage
• Positive sequence voltage
• Quadrature voltage
• Leading phase voltage

39
Memory Polarization

• Positive-sequence memorized voltage is used for
  polarizing
• Mho comparator (dynamic, expanding Mho)
• Negative-sequence directional comparator (Ground
  Distance Mho and Quad)
• Zero-sequence directional comparator (Ground
  Distance MHO and QUAD)
• Directional comparator (Phase Distance MHO and
  QUAD)
• Memory duration is a common distance settings
  (all zones, phase and ground, MHO and QUAD)

40
Memory Polarization
Static MHO characteristic (memory not established
or expired)
ZL
Dynamic MHO characteristic for a reverse fault
Dynamic MHO characteristic for a forward fault
Impedance During Close-up Faults
ZS
41
Memory Polarization
Static MHO characteristic (memory not established
or expired)
ZL
Dynamic MHO characteristic for a forward fault
RL
ZS
Memory PolarizationImproved Resistive Coverage
42
Choice of Polarization

• In order to provide flexibility modern distance
  relays offer a choice with respect to
  polarization of ground overcurrent direction
  functions
• Voltage polarization
• Current polarization
• Dual polarization

43
Ground Directional Elements

• Pilot-aided schemes using ground mho distance
  relays have inherently limited fault resistance
  coverage
• Ground directional over current protection using
  either negative or zero sequence can be a useful
  supplement to give more coverage for high
  resistance faults
• Directional discrimination based on the ground
  quantities is fast
• Accurate angular relations between the zero and
  negative sequence quantities establish very
  quickly because
• During faults zero and negative-sequence currents
  and voltages build up from very low values
  (practically from zero)
• The pre-fault values do not bias the developing
  fault components in any direction

44
Distance Schemes
Non-Pilot Aided Schemes (Step Distance)
Pilot Aided Schemes
Communication between Distance relays
No Communication between Distance Relays
45
Step Distance Schemes

• Zone 1
• Trips with no intentional time delay
• Underreaches to avoid unnecessary operation for
  faults beyond remote terminal
• Typical reach setting range 80-90 of ZL
• Zone 2
• Set to protect remainder of line
• Overreaches into adjacent line/equipment
• Minimum reach setting 120 of ZL
• Typically time delayed by 15-30 cycles
• Zone 3
• Remote backup for relay/station failures at
  remote terminal
• Reaches beyond Z2, load encroachment a
  consideration

46
Step Distance Schemes
Local
Z1
BUS
BUS
Z1
Remote
47
Step Distance Schemes
Local
End Zone
Z1
BUS
BUS
Z1
End Zone
Remote
48
Step Distance Schemes
Local
Z1
Breaker Tripped
BUS
BUS
Breaker Closed
Z1
Remote
49
Step Distance Schemes
Local
Z2 (time delayed)
Z1
BUS
BUS
Z1
Z2 (time delayed)
Remote
50
Step Distance Schemes

Z3 (remote backup)
Z2 (time delayed)
Z1
BUS
BUS
51
Step Distance Protection
52
Distance Relay Coordination
Local Relay
Remote Relay
53
Need For Pilot Aided Schemes
BUS
BUS
Local Relay
Remote Relay
Communication Channel
54
Pilot Communications Channels

• Distance-based pilot schemes traditionally
  utilize simple on/off communications between
  relays, but can also utilize peer-to-peer
  communications and GOOSE messaging over digital
  channels
• Typical communications media include
• Pilot-wire (50Hz, 60Hz, AT)
• Power line carrier
• Microwave
• Radio
• Optic fiber (directly connected or multiplexed
  channels)

55
Distance-based Pilot Protection
56
Pilot-Aided Distance-Based Schemes

• DUTT Direct Under-reaching Transfer Trip
• PUTT Permissive Under-reaching Transfer Trip
• POTT Permissive Over-reaching Transfer Trip
• Hybrid POTT Hybrid Permissive Over-reaching
  Transfer Trip
• DCB Directional Comparison Blocking Scheme
• DCUB Directional Comparison Unblocking Scheme

57
Direct Underreaching Transfer Trip (DUTT)

• Requires only underreaching (RU) functions which
  overlap in reach (Zone 1).
• Applied with FSK channel
• GUARD frequency transmitted during normal
  conditions
• TRIP frequency when one RU function operates
• Scheme does not provide tripping for faults
  beyond RU reach if remote breaker is open or
  channel is inoperative.
• Dual pilot channels improve security

58
DUTT Scheme

 
Zone 1
Bus
Bus
Line
Zone 1
59
Permissive Underreaching Transfer Trip (PUTT)

• Requires both under (RU) and overreaching (RO)
  functions
• Identical to DUTT, with pilot tripping signal
  supervised by RO (Zone 2)

60
PUTT Scheme
Rx PKP

Local Trip
Zone 2
OR
Zone 1
61
Permissive Overreaching Transfer Trip (POTT)

• Requires overreaching (RO) functions (Zone 2).
• Applied with FSK channel
• GUARD frequency sent in stand-by
• TRIP frequency when one RO function operates
• No trip for external faults if pilot channel is
  inoperative
• Time-delayed tripping can be provided

62
POTT Scheme

63
POTT Scheme
POTT Permissive Over-reaching Transfer Trip
End Zone
BUS
BUS
64
POTT Scheme
Remote Relay
Local Relay
65
POTT Scheme
Communications Channel(s)
POTT TX 1
A to G
POTT RX 1
POTT TX 2
B to G
POTT RX 2
POTT TX 3
C to G
POTT RX 3
POTT TX 4
Multi Phase
POTT RX 4
Local Relay
Remote Relay
66
POTT Scheme Current reversal example
Local Relay
Remote Relay
67
POTT Scheme Echo example
Open
Local Relay
Remote Relay
68
Hybrid POTT

• Intended for three-terminal lines and weak
  infeed conditions
• Echo feature adds security during weak infeed
  conditions
• Reverse-looking distance and oc elements used to
  identify external faults

69
Hybrid POTT
70
Directional Comparison Blocking (DCB)

• Requires overreaching (RO) tripping and blocking
  (B) functions
• ON/OFF pilot channel typically used (i.e., PLC)
• Transmitter is keyed to ON state when blocking
  function(s) operate
• Receipt of signal from remote end blocks tripping
  relays
• Tripping function set with Zone 2 reach or
  greater
• Blocking functions include Zone 3 reverse and
  low-set ground overcurrent elements

71
DCB Scheme
72
Directional Comparison Blocking (DCB)
End Zone
BUS
BUS
73
Directional Comparison Blocking (DCB) Internal
Faults
Local Relay
Remote Relay
74
Directional Comparison Blocking (DCB) External
Faults
Local Relay
Remote Relay
75
Directional Comparison Unblocking (DCUB)

• Applied to Permissive Overreaching (POR) schemes
  to overcome the possibility of carrier signal
  attenuation or loss as a result of the fault
• Unblocking provided in the receiver when signal
  is lost
• If signal is lost due to fault, at least one
  permissive RO functions will be picked up
• Unblocking logic produces short-duration TRIP
  signal (150-300 ms). If RO function not picked
  up, channel lockout occurs until GUARD signal
  returns

76
DCUB Scheme

 
77
Directional Comparison Unblocking (DCUB)
End Zone
BUS
BUS
78
Directional Comparison Unblocking (DCUB) Normal
conditions
Load Current
FSK Carrier
FSK Carrier
GUARD1 TX
GUARD1 RX
Local Relay
Remote Relay
GUARD2 TX
GUARD2 RX
NO Loss of Guard
NO Loss of Guard
NO Permission
NO Permission
Communication Channel
79
Directional Comparison Unblocking (DCUB) Normal
conditions, channel failure
Load Current
FSK Carrier
FSK Carrier
GUARD1 TX
GUARD1 RX
Local Relay
Remote Relay
GUARD2 TX
GUARD2 RX
Communication Channel
80
Directional Comparison Unblocking (DCUB) Internal
fault, healthy channel
FSK Carrier
FSK Carrier
GUARD1 TX
GUARD1 RX
Local Relay
Remote Relay
GUARD2 TX
GUARD2 RX
Communication Channel
81
Directional Comparison Unblocking (DCUB) Internal
fault, channel failure
FSK Carrier
FSK Carrier
GUARD1 TX
GUARD1 RX
Local Relay
Remote Relay
GUARD2 TX
GUARD2 RX
Communication Channel
82
Redundancy Considerations

• Redundant protection systems increase
  dependability of the system
• Multiple sets of protection using same protection
  principle and multiple pilot channels overcome
  individual element failure, or
• Multiple sets of protection using different
  protection principles and multiple channels
  protects against failure of one of the protection
  methods.
• Security can be improved using voting schemes
  (i.e., 2-out-of-3), potentially at expense of
  dependability.
• Redundancy of instrument transformers, battery
  systems, trip coil circuits, etc. also need to be
  considered.

83
Redundant Communications
End Zone
BUS
BUS
AND Channels
OR Channels
POTT Less Reliable
POTT More Reliable
Communication Channel 1
DCB More Secure
DCB Less Secure
Communication Channel 2
More Channel Security
More Channel Dependability
84
Redundant Pilot Schemes
85
Pilot Relay Desirable Attributes

• Integrated functions
• weak infeed
• echo
• line pick-up (SOTF)
• Basic protection elements used to key the
  communication
• distance elements
• fast and sensitive ground (zero and negative
  sequence) directional IOCs with current, voltage,
  and/or dual polarization

86
Pilot Relay Desirable Attributes

• Pre-programmed distance-based pilot schemes
• Direct Under-reaching Transfer Trip (DUTT)
• Permissive Under-reaching Transfer Trip (PUTT)
• Permissive Overreaching Transfer Trip (POTT)
• Hybrid Permissive Overreaching Transfer Trip (HYB
  POTT)
• Blocking scheme (DCB)
• Unblocking scheme (DCUB)

87
Security for dual-breaker terminals

• Breaker-and-a-half and ring bus terminals are
  common designs for transmission lines.
• Standard practice has been to
• sum currents from each circuit breaker externally
  by paralleling the CTs
• use external sum as the line current for
  protective relays
• For some close-in external fault events, poor CT
  performance may lead to improper operation of
  line relays.

88
Security for dual-breaker terminals
Accurate CTs preserve the reverse current
direction under weak remote infeed
89
Security for dual-breaker terminals
Saturation of CT1 may invert the line current as
measured from externally summated CTs
90
Security for dual-breaker terminals

• Direct measurement of currents from both circuit
  breakers allows the use of supervisory logic to
  prevent distance and directional overcurrent
  elements from operating incorrectly due to CT
  errors during reverse faults.
• Additional benefits of direct measurement of
  currents
• independent BF protection for each circuit
  breaker
• independent autoreclosing for each breaker

91
Security for dual-breaker terminals

• Supervisory logic should
• not affect speed or sensitivity of protection
  elements
• correctly allow tripping during evolving
  external-to-internal fault conditions
• determine direction of current flow through each
  breaker independently
• Both currents in FWD direction ? internal fault
• One current FWD, one current REV ? external fault
• allow tripping during all forward/internal faults
• block tripping during all reverse/external faults
• initially block tripping during evolving
  external-to-internal faults until second fault
  appears in forward direction. Block is then
  lifted to permit tripping.

92
Single-pole Tripping

• Distance relay must correctly identify a SLG
  fault and trip only the circuit breaker pole for
  the faulted phase.
• Autoreclosing and breaker failure functions must
  be initiated correctly on the fault event
• Security must be maintained on the healthy
  phases during the open pole condition and any
  reclosing attempt.

93
Out-of-Step Condition

• For certain operating conditions, a severe
  system disturbance can cause system instability
  and result in loss of synchronism between
  different generating units on an interconnected
  system.

94
Out-of-Step Relaying

• Out-of-step blocking relays
• Operate in conjunction with mho tripping relays
  to prevent a terminal from tripping during severe
  system swings out-of-step conditions.
• Prevent system from separating in an
  indiscriminate manner.
• Out-of-step tripping relays
• Operate independently of other devices to detect
  out-of-step condition during the first pole slip.
• Initiate tripping of breakers that separate
  system in order to balance load with available
  generation on any isolated part of the system.

95
Out-of-Step Tripping
The locus must stay for some time between the
outer and middle characteristics
When the inner characteristic is entered the
element is ready to trip
Must move and stay between the middle and inner
characteristics
96
Power Swing Blocking

• Applications
• Establish a blocking signal for stable power
  swings (Power Swing Blocking)
• Establish a tripping signal for unstable power
  swings (Out-of-Step Tripping)
• Responds to
• Positive-sequence voltage and current

97
Series-compensated lines

• Benefits of series capacitors
• Reduction of overall XL of long lines
• Improvement of stability margins
• Ability to adjust line load levels
• Loss reduction
• Reduction of voltage drop during severe
  disturbances
• Normally economical for line lengths gt 200 miles

98
Series-compensated lines

• SCs create unfavorable conditions for protective
  relays and fault locators
• Overreaching of distance elements
• Failure of distance element to pick up on
  low-current faults
• Phase selection problems in single-pole tripping
  applications
• Large fault location errors

99
Series-compensated linesSeries Capacitor with MOV
100
Series-compensated lines
101
Series-compensated linesDynamic Reach Control
102
Series-compensated linesDynamic Reach Control
for External Faults
103
Series-compensated linesDynamic Reach Control
for External Faults
104
Series-compensated linesDynamic Reach Control
for Internal Faults
105
Distance Protection Looking Through a Transformer

• Phase distance elements can be set to see beyond
  any 3-phase power transformer
• CTs VTs may be located independently on
  different sides of the transformer
• Given distance zone is defined by VT location
  (not CTs)
• Reach setting is in ?sec, and must take into
  account location ratios of VTs, CTs and voltage
  ratio of the involved power transformer

106
Transformer Group Compensation

• Depending on location of VTs and CTs, distance
  relays need to compensate for the phase shift and
  magnitude change caused by the power transformer

107
Setting Rules

• Transformer positive sequence impedance must be
  included in reach setting only if transformer
  lies between VTs and intended reach point
• Currents require compensation only if
  transformer located between CTs and intended
  reach point
• Voltages require compensation only if
  transformer located between VTs and intended
  reach point
• Compensation set based on transformer connection
  vector group as seen from CTs/VTs toward reach
  point

108
Distance Relay Desirable Attributes

• Multiple reversible distance zones
• Individual per-zone, per-element characteristic
• Dynamic voltage memory polarization
• Various characteristics, including mho, quad,
  lenticular
• Individual per-zone, per-element current
  supervision (FD)
• Multi-input phase comparator
• additional ground directional supervision
• dynamic reactance supervision
• Transient overreach filtering/control
• Phase shift magnitude compensation for distance
  applications with power transformers

109
Distance Relay Desirable Attributes

• For improved flexibility, it is desirable to have
  the following parameters settable on a per zone
  basis
• Zero-sequence compensation
• Mutual zero-sequence compensation
• Maximum torque angle
• Blinders
• Directional angle
• Comparator limit angles (for lenticular
  characteristic)
• Overcurrent supervision

110
Distance Relay Desirable Attributes

• Additional functions
• Overcurrent elements (phase, neutral, ground,
  directional, negative sequence, etc.)
• Breaker failure
• Automatic reclosing (single three-pole)
• Sync check
• Under/over voltage elements
• Special functions
• Power swing detection
• Load encroachment
• Pilot schemes

111
Questions?