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Distance Relaying Flashcards

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1
Q

Impedance in regards to distance relaying

A

▪︎Used to tell between faults and normal system conditions or between faults in a specific area and those elsewhere on the system
▪︎Done by limiting relay operation to a certain range of impedance

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2
Q

How is impedance on a transmission line determined

A

▪︎Type of conductor
▪︎Size of conductor
▪︎Spacing between conductors
▪︎Length of the line

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3
Q

Basic distance relaying principles

A

▪︎In a Phase to phase fault current would be virtually limitless without the impedance of the line
▪︎The further the distance between the fault location and the source of fault current the greater the impedance will be
▪︎Faults reduce power system voltage, based on severity and location
• During a fault voltage is highest at generation but drops to near 0 at the fault
▪︎These characteristics allow distance relays to measure the distance to the fault

Ohm’s law: V/I = Z
during a fault: Z of a transmission line is =V/I
If the Z/km of the line is known we can calculate the distance to the fault

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4
Q

What are zones of protection in regards to distance relays

A

▪︎Protection of a line uses multiple zones of protection.
▪︎These cover different lengths of the line and use different times for tripping
▪︎Typically 3 Zones
• Zone 1: under-reaching 80-90% of the line, instantaneous
• Zone 2: overreaching 120-150% of the line, time delayed
• Zone 3: overreaching 150-200% of the line, time-delayed. Can also look reverse

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5
Q

Zone 1

A

▪︎Generally 80-90% of the line
▪︎Instantaneous
▪︎Direct under-reaching
▪︎Cannot be set to 100% because it may trip for a fault beyond the line
▪︎80-90% used to account for possible relay inaccuracies and voltage and current transformer errors

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6
Q

Zone 2

A 
▪︎Reaches beyond the next substation
▪︎120-150%
▪︎Permissive over-reach
▪︎Time delayed to prevent mis-operation
▪︎Protects remaining portion of the line
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7
Q

Zone 3

A

150-200%

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8
Q

3 terminal line relay settings

A

Zone 1 set to 80% of the nearest terminal

Zone 2 set to 150% of the furthest terminal

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9
Q

Rx diagram

A

▪︎Used to visually show a relays characteristics and how impedance is seen by a distance relay
▪︎Impedance consists of both resistance(R, shown on the x axis) and reactance(X, shown on the y axis)
• Is represented by a phasor plotted on the Rx diagram
The circle represents the “reach” or characteristic of a zone

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10
Q

Rx Diagrams: reach

A

Distance in ohms along a line defined by the relay angle in the forward direction at which the relay begins to develop contact closing torque

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11
Q

Rx diagram; relay angle

A

Angle defined by the line through the origin and the center of the relay characteristics
▪︎Normally factory set at 80%

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12
Q

Rx diagram: off set

A

▪︎Distance in ohms of the reverse reach
▪︎Typically not seen in zone 1 or 2 relays
▪︎Zone 3 relays can provide time delayed bus protection

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13
Q

Sources

A

Because distance delays function via Ohm’s Law current and potential sources are needed
▪︎Current input is from bidding CTs on the associated PCB and determine the direction of the relay
▪︎Voltage inputs deter mines the point from which the impedance is measured
• On a Main/Aux bus it’s from the M/B PTs
• On breaker and a half or ring bus it’s from the line CVTs

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14
Q

Types of distance relays

A

▪︎Impedance
▪︎Reactance
▪︎Admittance
▪︎Compensator

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15
Q

Impedance (balance beam) relay

A

▪︎There first style of distance relay used by BPA (Westinghouse HZ)
▪︎Two opposing coils affect the balance beam
• Voltage coil energized by bus voltage
• Current coil energized by line current
▪︎Normally the strength of the voltage coil is greater then that of the current coil =>the contacts are restrained open
▪︎When a fault occurs, voltage goes down and current goes up, overcoming the restraint voltage and closing the contacts
▪︎The ratio of voltage to current that will trip the balance beam is the “trip impedance”
▪︎This type of relay is non-directional
• A separate high speed directional unit is added

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16
Q

Reactance relay

A

▪︎Developed to counter that impedance relays may fail if there is high fault resistance
▪︎Ignores resistance and only looks at transmission line reactance to determine if a fault occurred

▪︎Most common was the GE GCX relay
▪︎Required directional supervision
▪︎Functioned on the induction cup principle

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17
Q

Admittance relay

A

▪︎Admittance (MHO) is the opposite of impedance (OHM)
• Impedance measures the opposition to current flow
• Admittance is a measure of how permissive a conductor is to current flow
• Mathematically, admittance is the inverse of impedance
▪︎Early MHO relay was the GE GCY
• MHO element 4-pole induction disk
• No added directional unit needed
• Each case contained 3 elements for 3 zone
• Storage timing relays needed for zone 2 and 3

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18
Q

Compensator relays

A

▪︎Most common electro mechanical relay on the system
▪︎KD - 3 phase single zone relay
• Voltage operated relay, operating voltage is a combo of voltages from three PTs and current is from the line CTs
• Contains 2 3-phase units, one for phase-place faults and the other for 3- phase faults
• 3 phase currents and voltages are supplied to the “compensators” for each unit
• Compensators are air- gap transformers
•Primary is supplied by current from CTs
•Secondary is supplied by voltage from PT

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19
Q

Timing relays for distance relays

A

▪︎Directional distance relays operate instantaneously for phase- phase and 3 phase faults that occur in the correct direction and with in the zone of protection therefore a time delay relay is needed for zones 2&3

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20
Q

Loss of potential

A

▪︎It’s a relay trap for pilot and distance delays
▪︎KD relays are especially sensitive
▪︎During normal operations
• The KD relay is retained by potential
• Loss of potential will cause the relay to lose restraining torque and trip contracts will close
▪︎KC over current fault detectors are used to ensure there is a fault but need to be set very low, so high current might be seen as a fault
• Can not be depended on for preventing a trip during all LOP situations

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21
Q

Standard line protection relay package with electromechanical relays

A

▪︎Directional ground relays (phase to ground protection)
•Instantaneous: set to under-reach, keys a pilot relay direct trip
•Time delayed: lightly set to over-reach, let’s a pilot relay permissive trip
▪︎Distance relays (phase- phase protection)
•Zone 1: under-reach ~80% of the line, keys direct trip
•Zone 2,3,4: time delayed over-reach, keys permissive trip
▪︎Pilot relaying: MWTT, mirror bit, CC etc.
▪︎Reclosing relay
▪︎Optional:
•KC fault detectors - verifies fault for LOP
•Multi-phase fault detectors for reclose blocking
•Switch on to fault relays
•JM annunciator

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22
Q

SEL-121G

A

▪︎Early SEL relay
▪︎Contains:
• O/C fault detectors
• Directional ground relays
• Distance relays with impedance measuring
• Reclosing
▪︎Is the equivalent to 3 KD relays, an IRD-9, a KRD, reclosing relay, and a KC fault detector
▪︎A computer is needed to access fault event info

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23
Q

SEL-121G inputs

A

▪︎Current from PCB bushing CTs
▪︎Relay potential is usually from bus PTs
▪︎Polarizing potential and current from the usual sources
• Can also develop its own negative sequence polarizing

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24
Q

SEL-121G reports

A

▪︎Long reports
• Gives a cook plate list of voltages and currents with a sample every 1/4 cycle for 11 cycles
• Indicates when relay inputs and outputs are asserted as well as the operation of various relay elements
• Instantaneous trips take only one event report
• Trip with a time delay >12 cycles takes 2 event reports
▪︎Short reports
• Do not list quarterly samples
▪︎Both reports
• Event 1 is the most recent event and 12 is the oldest
• All event history is lost when the relay is powered down

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25
How are SEL-121G connected
▪︎Normally installed with redundancy • Each has a separate connection for voltage and current sources ▪︎Allows one relay to be OOS for maintenance
26
SEL-121G outputs
``` ▪︎Has 7 output contracts • Trip • Close • Alarm • 4 programmable outputs ```
27
SEL-121G targets
▪︎EN: relay is in service ▪︎Zone 1, 2, 3 phase: which phases are affected for distance relay trip ▪︎Zone 1G: instantaneous ground O/C ▪︎Zone 2G: permissive ground or step time ground O/C ▪︎Zone 3G: step time ground O/C ▪︎51N: time delay ground O/C ☆Targets are cumulative, in not reset multiple faults will cause multiple LEDs to light fir different types of faults • Example: a zone 1 fault is seen by zones 2 &3 so all 3 phase LEDs will be lit
28
SEL-121G serial interfaces
▪︎Connectors are labeled PORT 1 & PORT 2 for serial data interfaces • Generally PORT 1 is for remote communications • PORT 2 is for local comms via a computer or SEL-PRTU or SEL-DTA, for local operator interface or transducer O/P
29
What it's a PRTU
▪︎Protective Relay Terminal Unit ▪︎Is used as a multiplexed between a modem and the SEL relays •Can address upto 8 •Multiple PRTUs can be linked
30
What is pilot relaying
▪︎A generic term for communication systems where 2 or more terminals send and recieve info to accomplish instantaneous tripping • Provides a high level of protection with distance relays to cover 100% of the line • Increases system stability, continuity of service, minimum damage to lines& equipment, andrapid reclosing
31
Types of pilot relaying
``` ▪︎Microwave ▪︎Fiber optics ▪︎Power-line carrier ▪︎Pilot wire ▪︎UHF radio ▪︎Leased line ▪︎Mirror bit (relay-to-relay) ```
32
Categories of pilot relaying
``` ▪︎Current comparison ▪︎Directional comparison blocking ▪︎Carrier current transfer trip ▪︎Microwave transfer trip ▪︎Line differential ```
33
Pilot wire
▪︎The system is hardwired between terminals ▪︎Used on short runs ▪︎Requires complex auxiliaries for secure operation, like sequence filters and saturating transformers ▪︎Most have been removed from the system and replaced by microwave relays ▪︎Most were current comparison- type schemes
34
Fiber optics
▪︎Becoming more prevalent on newer remote tripping schemes ▪︎Uses a "word bit" stream, like "all ok" or "trip" instead of com tones like with microwave ▪︎Signals are sent by light over glass fibers
35
Carrier current
▪︎High frequency current superimposed on power lines ▪︎Only a few on the system, usually associated with customers, but most have upgraded ▪︎Communications between line terminals by utilizing a single phase of the associated line ▪︎2 types on the system • Carrier current transfer trip • Directional comparison blocking schemes
36
Carrier current tones
▪︎Each terminal has a transmitter and reciever for 1 tone, that shifts from guard to trip ▪︎Communication equipment consists of transmitters, recorders, tuning units, coupling capacitors, drain coils, and wave traps at each end ▪︎During a fault in the zone of protection relays operate causing the local breaker to open directly and shifting there transmitter tone from guard to trip frequency ▪︎At the remote terminal, the reciever contacts trip when the tone changes to the trip frequency, which them trips the breaker ▪︎Frequency for guard and trip are deteriorated by a few 100Hz
37
Carrier current transfer trip
▪︎Older use of carrier tones ▪︎Used as a direct tripping scheme: any line protective relay that slides TN will generate a trip output via carrier signal to remote terminal ▪︎Deemed unreliable because the signal id on the transmission line that can be damaged by the fault ▪︎Utilizing 1 tone separated by a few 100Hz from guard to trip caused security problems
38
Carrier current blocking scheme
▪︎More common use of carrier current ▪︎Consists of the usual 3 Zone distance relays and directional ground relays, plus aux relays ▪︎Blocks faults that are external to the protected line ▪︎Absence of the block signal permits tripping for internal faults ▪︎Less susceptible to false tripping for a fault on the line with carrier signals as a trip will only occur without the blocking signal
39
Microwave
▪︎Most common communication system at BPA ▪︎Advantages: • Signal is completely independent of transmission lines • It is allegedly prevalent on the system • More sophisticated comms are possible • Increased security for transfer trip schemes ▪︎Signal can be subject to occasional fading due to atmospheric conditions therefore best suited transfer trip schemes ▪︎Beamed by parabolic antennas, therefore generally require line-of-sight
40
Microwave tones
▪︎Basic function and operation is essentially the same as CCTT with significant differences in principle and equipment ▪︎Uses dual-channels in the transfer trip scheme • 2 tone transmitters and 2 tone receivers at each terminal • Known as Tone 1 (TT1 or TR1) and Tone 2 (TT2 or TR2) • Provides added security against false trips because both transmitters shift from Guard to Trip and both trip frequencies micr be recieved for three TT to take place ▪︎During normal operations, two Tone transmitters send continuous Guard signals, during a fault in three protected zone a PR trips the local breaker and shifts both tone transmitters to Trip (keying) ▪︎at the remote terminal the two tone receivers contacts operate due to the shift from Guard to Trip • then depending on the scheme, picks up an aux microwave relay that trips the breaker and drops a target
41
Direct under-reach
▪︎Also called Direct Transfer Trip (DTT) ▪︎Is keyed by under-reaching relays at one or both of the terminals ▪︎Can be keyed by; zone 1 distance relays and instantaneous directional ground relays ▪︎requires both Tone 1 and Tone 2 signals to be sent ▪︎A timer on the transfer trip panel at the local terminal maintains the tones for several milliseconds after the protective relays drop out to ensure the DTT signal gets through to the remote terminal (~60mS)
42
Permissive over-reach
▪︎Also called Permissive Transfer Trip (PTT) or Permissive Overreach Transfer Trip (POTT) ▪︎It is best described as a directional comparison scheme ▪︎PTT only requires a single signal (Tone 2) but also needs "permission" from the line protective relay to trip ▪︎Relays that key PTT are usually set up to overreach the remote terminal • Zone 2 or 3 phase relays • Sensitively set instantaneous ground relays ▪︎When a permissive relay is picked up, it keys a permissive tone (TT2) to the remote terminal ▪︎The trip occurs when the permissive relay is picked up along with receiving the Tone 2 ▪︎Zone 2 directional distance relays and timed directional ground relays perform 2 functions on the initial sensing of a fault • Start a timer • Key Tone 2 transmit to the remote end
43
How does permissive trip work with an open- ended line
▪︎When the local PCB is open, a permissive tone is keyed at the remote terminal ▪︎Uses the 52b contract in the PCB to key Tone 2, this is b- repeat logic ▪︎For a breaker- and-1/2 pr ring bus, the 52b switch for all PCBs used to isolate the line are in series • All PCBs must be open to be enabled ▪︎If the protective relay(zone 2) picks up at the remote end, a trip will occur ▪︎This allows for high speed tripping instead of time delay for the entire length of the line
44
Explain echo back
▪︎Used where letting a continuous tone 2 for an open- ended line is not the best protection, like for single pole tripping ▪︎A permissive echo is used ▪︎'B' repast logic enables the echo scheme instead of sending a continuous tone 2 ▪︎The closed terminal sees a zone 2 fault, sends the tone 2 to the open terminal which then "echos" back the signal to the closed terminal allowing an instantaneous trip ▪︎Many 500kV lines, including SEL relays have echo back capabilities
45
Echo back on SEL-321
▪︎Reverse zone 3 phase and zone 3 ground relays are used tol block echo back tones if the fault is behind the relay ▪︎This scheme covers the open remote terminal condition as the reverse delay can't operate if the PCB is open (no relay current) ▪︎When a permissive tone is received a a reverse fault is not detected, the tone 2 is echoed back
46
How is TT connected for 3 terminal lines
▪︎More complicated then 2 terminal lines ▪︎Requires 2 sets of TT gear • One station will have one set of TT connected to the remote end, the other is connected to the tap • The 2 sets out equipment at the one station are also connected together, is said to be connect back- to- back
47
DTT for 3 terminal lines
▪︎relays at the remote station or the tap key a direct transfer trip to the "back to back" station which will trip that PCB and send a DTT to the other station ▪︎If relays at the "back to back" station see a zone 1 fault, the local relay will trip and send a DTT to both the other stations
48
3 terminal permissive trip
▪︎More complicated then DTT ▪︎Requires more connections at the back-to-back terminal ▪︎All 3 stations need to see the fault in front of them in order to trip • The remote and tap stations work the same as the would for a 2- terminal line • At the back to back station, when a permissive relay picks up, it will recieve a PTR from one of the other stations then send a PTT to the other station ▪︎Uses 52b switches to enable a permissive tone for outages same as a 2- terminal
49
3 terminal relay protection
▪︎2 basic configurations ▪︎1st and most common is to have one set of relays at each terminal looking forward at the line • Common ether the tapped line is long • Zone 1 set to ~85% of these shortest line • Zone 2 set to overreach the longest line • These settings are more difficult with a great difference in length for the 3 lines ▪︎Another type is to use 2 sets of relays at the tap, each looking in different directions • TT is connected back to back at the tap • Gives more definitive coverage by treating the line sections at different lines • Both direct and permissive on one section function as they would for a2- terminal line but tone is sent to the 3rd terminal too
50
Current reversal logic
▪︎Permissive trip requires time coordination to account fir relay pick up and drop out times and current reversals on parallel lines • With out coordination it's possible to "over trip" the unfaulted lines
51
SEL-321
▪︎Improved from three SEL-121G by adding a human-machine interface • Outputs from all Protection elements, logic schemes, etc can be physically programmed by the user ▪︎Phase distance relay has up to 4 zones of protection ▪︎Storage phase over current supervision is available for each zone to act as fault detectors ▪︎Capabilities increased to 16 inputs and 32 programmable outputs with 6 setting groups • Each setting group is a complete relay setting package and only 1 group can be used at a time ▪︎Includes internal isolating transformers that correct for CT ratio matching and phase- shifting transformer errors ▪︎Can be linked directly to SCADA for trip and close
52
SEL-321 reports
▪︎4 typesof event reports ▪︎40 of the most recent history events are stored in nonvolatile memory • Not lost if there relay is powered down • These are summary event reports that include the date, time, event type, faculty location, setting group, and targets ▪︎Auto summary report with abbreviated info • Most often communicated with the printer • Can be accessed remotely or locally on the display • Most useful to operators
53
Mirror bit communications
▪︎Allows protective relays to talk directly to each other ▪︎Each relay has an 8- bit transmitter and reciever • Each bit can be configured differently • Usually transmitting a 1 or 0 depending on how it is configured ▪︎The signal is "mirrored" at the remote terminal ▪︎When a fault occurs it changes the signal to the opposite of the normal ▪︎The new signal is then mirrored at the remote terminal indicating proper output ▪︎The commission link can be analog or digital • Though analog is usually too slow to be useful so mainly use digital
54
SEL-421
▪︎Improved upon from the 121/321, closer to a PLC ▪︎Has more SCADA- like functions such as, breaker/ equipment monitoring and operation via 32 local control switches, 32 remote control switches, 32 latching switches and programmable display messages
55
SEL-421 front panel display
▪︎The LCD display shows events, metering, settings, and self- test info • Is controlled by the navigation push buttons, auto- messages the relay generates, and user programmed analog and digital display points • The rotating display scrolls through alarm points, display points, and metering screens ▪︎LED Target's ENABLED: unit is energized, normal condition TRIP: lights for any trip event INST: any instantaneous trip TIME: any time delayed trip COMM: any communication assisted trip (DTR, PTR) SOTF: Switch On To Fault Zone 1,2,3,4: which zone the fault was in Phase 1,2,3: which phases were affected GROUND: if the ground was affected 50: instantaneous over current fault 51: time delay over current fault 79 RESET: recloser is reset (ready to operate) 79 LOCKOUT: recloser locked out (unable to operate)
56
RFL 6750
▪︎MicrowaveTransfer trip unit that operates on the frequency shift- keying principle • Continually sends a Guard tone • When keyed shifts to a Trip tone • Recieving unit detects the shift and closes its output contacts ▪︎Security depends on the ability to prevent fault trips during adverse a atmosphere conditions ▪︎Has echo back logic controlled by single pole relays ▪︎Has remote trouble alarm logic to detect problems at the remote terminal ▪︎Meets 3 different TT needs • Operates with standard electromechanical relays • Able to be used with single-pole tripping • Designed to meet the needs of RAS
57
RFL 6750 display and how to cutouta
▪︎Tone recieve and transmit counters, show the number of time keyed since last reset ▪︎Outrage timer will visibly operate during an outage ▪︎LCD counters reset with three white button ▪︎PTT & DTT target lights are reset with reset button ▪︎To cutout: take the "Tone Test Switch" TTS to off
58
Iniven PTR1000
Next generation TT I'm wide use at BPA Because of the TT capabilities of microprocessor relays, a conventional tone test panel is not needed Generally only installed with a counter panel Contains operation counters, jacks, and aTT C/O switch (TTCS) Uses key shifting principle Designed to use standard audio comm media Low- bandwidth radio, microwave, and hardwire Has optional fiber transceivers 2 solid state output modules Tone 1 reciever operates one module. Tone 2 reciever operates the other Each has 6 N.O. outputs Green LEDs light when reciever logic is keying the relay
59
What it's a minute tone test switch
▪︎4 position switch: ON, OFF, TEST TONE 1, TEST TONE 2 ▪︎Cuts in and out the transmitter and reciever and provides tone 1 & 2 keying signals for testing ▪︎Provides supervision for for solid state outputs ▪︎Does not provide sufficient air- gap • for isolation use the Transfer Trip Cutoff Switch for tagging
60
RFL9300
▪︎Digital charge comparison system including an integrated relay/ comm terminal for high speed protective relaying ▪︎Measures instantaneous values of each phase and ground current once every half- millisecond ▪︎Local and remote charges are compared, with polarity ▪︎If net charge differs from 0 by a certain amount (bias) a fault is declared and trips are sent ▪︎Often used near powerhouses ▪︎Often coupled with a SEL-321 for backup protection ▪︎Charge comparison can be disabled and the RFL9300 used only as a comm path to trip Via the 321
61
Relay traps for distance relays
All impedance relays are service to Loss of Potential
62
Common causes for Loss of Potential
▪︎Relay potential ACB inadvertently operated ▪︎Inadvertent break in relay potential circuit due to work in the substation ▪︎Switching
63
Bus outages in terms of LOP
▪︎Most common cause of LOP ▪︎When potential is lost voltage resistant is lost so a KD relay will send a DTT tol the remote terminal • Cut out KD relays prevent inadvertent • TT will need to be cut out to cut out the KD relay then cut back in • If TT is left Off the DTT is interrupted but so is PRT which is required for instantaneous tripping for the entire length of the line
64
LOP in electronic relays
▪︎LOP feature needs to be enabled for SEL relays to block for a LOP ▪︎Some single- pole relays are susceptible to false tripping for LOP ▪︎Unless stated in the Station Instructions contact SPC if there is an expected LOP ▪︎For SEL-321 if LOP is enabled and there are a set of non- directional high set O/Cs for relay protection, during a LOP event only non-directional phase and ground over current protection remains • No phase to phase or directionality
65
Other relay traps for electronic relays
▪︎Can lose fault history if the relay is powered down, • Use RSS to disable instead ▪︎Many relays can be accessed remotely ▪︎Older design standards used reclosing only with on set so if that set its out of service for maintenance you will not have reclosing
66
Distance relaying problems
``` ▪︎Close in 3 phase fault ▪︎Apparent impedance ▪︎Load encroachment ▪︎Fault resistance ▪︎Series compensated lines ▪︎Power swings ```
67
Fault (arc) resistance
▪︎Most severe on short lines ▪︎Difficult for ground resistance to detect ▪︎Solutions: • Tilt towards R axis • Use wide quadrilateral characteristics • Use over current relays for ground faults
68
Close in 3 phase fault
▪︎Low/ no voltage to make impedance measurements • Use memory action ▪︎Use non-dir, inst, OC relay (50-DLF) ▪︎Use SOTF logic
69
Series compensated lines
▪︎Series caps electrically shorten lines ▪︎Z1 = 80-90% of the compensated line impedance • Must not overreach the remote bus with caps I/S ▪︎Z2 = 125% of uncompensated line impedance • Must provide direct tripping for any line fault with caps bypassed
70
Power swings
▪︎Can cause false trips of 3 phase element ▪︎Option to: • Block on swing (out of step blocking) • Trip on swing (out of step tripping)
71
Apparent impedance
▪︎On 3 terminal lines with in- feed • Fault resistance looks like an apparent impedance • Important to account for on very short pr imbalanced legs ▪︎Results in: • Short Z1 reach - 85% of the shortest leg • Long Z2 reach with time delay- 125% of the furthest leg ▪︎Needs pilot relaying

Step 5 (2 decks)

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