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2. ABB Protective Relay School Webinar Series
Disclaimer
ABB is pleased to provide you with technical information regarding protective relays. The material included is not intended to be a complete presentation of all potential problems and solutions related to this topic. The content is generic and may not be applicable for circumstances or equipment at any specific facility. By participating in ABB's web-based Protective Relay School, you agree that ABB is providing this information to you on an informational basis only and makes no warranties, representations or guarantees as to the efficacy or commercial utility of the information for any specific application or purpose, and ABB is not responsible for any action taken in reliance on the information contained herein. ABB consultants and service representatives are available to study specific operations and make recommendations on improving safety, efficiency and profitability. Contact an ABB sales representative for further information.

3. Bus Protection Fundamentals
ABB Protective Relay School Webinar Series
Bus Protection Fundamentals
Jack Chang, RTM, CA
Sept 09, 2014

4. Presenter
Jack Chang is the regional technical manager for ABB Inc. in the Substation Automation Products BU serving customers in Canada and northern regions. He provides engineering, commissioning and troubleshooting support to customers applying ABB’s high-voltage protective and automation devices. Prior to joining ABB, Jack worked as a substation P&C project engineer in two specialized consulting firms and also as an engineering contractor to a public owned utility in their transmission expansion and upgrade projects. Jack is a registered professional engineer in the province of Alberta, Canada.
Jack Chang
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5. Learning objectives Types of bus configurations
Current transformer characteristics and their effect on bus protection
Types of bus protection schemes
Modern Numerical Bus Protection Features
Questions:
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6. Why bus protection? Different configuration and design
Usually very robust, high current faults
Need to clear quickly
A Delayed bus trip leads to:
Network instability pole slip of nearby generators
Possible system collapse
Bigger fault related damages & risk to human life or injury
Many bus faults caused during maintenance (eg. Arc flash)
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7. Bus fault protection Easy to detect because of robust nature
Easy to protect for internal faults (87B)
Summation of currents not equal to zero for internal fault
External faults can cause current transformer saturation which results in unwanted differential currents
Infrequent, but must be cleared with high speed
Substation is well shielded
Protected environment
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8. Bus configurations Single bus * Main and transfer bus
Double bus, single breaker *
Double bus, double breaker
Breaker and a half *
Ring bus *
* Dentoes commonly seen configuration in NAM
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9. Single bus
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10. Main and transfer bus
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11. Double bus single breaker
Typically seen in GIS SWGR
Lots of feeders, seen mainly in big industrail plants and utility distribution stations
Some types of dynamic zone switching scheme must be in place (modern low impedance bus differential IEDs)
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12. Double bus, double breaker
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13. Breaker and one half
Typically seen in HV/EHV transmission and major power plant switching stations, quite popular in North America
Dedicated bus protection each of two main bus bars
Bus stubs are located in the line/transformer/reactor zones and no dedicated bus protection required.
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14. Ring bus
Popular in North America (72, 138, 230 KV). Cheaper design than 1-1/2. same number of links as the number of breaker. less reliable.
Usually no dedicated bus bar protection as the bus stubs are in the zones of line/transformer/reactor etc with redundant A/B
All bus protection zones should be overlapped
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15. Main-Tie-Main
Most popular for MV (15, 25 KV) small industrial/utrility distribution systems
Dedicated bus diff. for each bus if so designed
blocking scheme via communciation
Some types of bus transfer scheme may be designed
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16. Some issues Availability of overlapping protection zones (CTs)
Blind or end zone protection
Will loads or sources be switched from one bus to the other
Current transformer switching from one zone to another
Open circuit current transformers
CT Saturation
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17. Zones of protection Station B Station A Station C Station D G G G
Bus protection
Station D M
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18. Zones of protection CT for Green Zone CT for Blue Zone Green Zone
Dead tank breaker, two CTs
Occurrence so rare as the fault is inside the tank. Bus fault beside the CT and Breaker
- BKR closed: both zones should operate in high speed to clear the fault
- BKR open: tripping of blue relay is unnecessary. To avoid over tripping with breaker operated normally open, logics can be made to remove the two CTS to the bus relay when the breaker is open
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19. Zones of protection CT for Green Zone CT for Blue Zone Green Zone
Live tank breaker, single CT
Blue relays open the breaker, but Green relays don’t. That fault is still live
Local breaker failure protection could be used as back-up if time delay is acceptable.
Very rare in likelihood of occurrence
* A faster solution is to “disconnect” the CT to the green relay when the breaker is open. Tripping for green relay will delay slightly but definitely faster than BF backup time.
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20. Ratings of concern for bus protection
Current transformer
Ratings of concern for bus protection
Ratio: 200/5, 1200/5, …500/1
Burden capability: VA burden
Accuracy Class: C800, K200, T400
Knee point, saturation voltage (can be derived from chart in C class CTs)
CT availability
IEEE Standard C (R2003), IEEE Standard Requirements for Instrument Transformers
IEEE Standard C , IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes
Accuracy Definition: For relaying CTs, ANSI/IEEE defines a limit of 10 percent for the ratio error for a steady-state, symmetrical (no dc offset)
Bus faults tend to saturate the CT closest to the fault (external fault)
secondary current equal to 20 times rated secondary current at the standard rated burden
C: Leakage flux neglegiable. Eg. Multi-ratio bushing, window type CTs. CT Excitation chracteristic can be used to determine CT performance
T: CT ratio error must be derived from test.
CT transient behavior must take into account the system X/R ratio (dc offset, deteriorating) and remnant flux (derate rated votlage). Simply size CT according to the steady-state rating is not sufficient to avoid saturation,
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21. ANSI current transformer accuracy class
Example 1200/5 C800
This current transformer will deliver 800 volts on its secondary when it is connected to a standard burden and 20 times rated current is flowing in the primary winding without exceeding 10% ratio error.
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22. Standard chart for class C current transformers
ANSI accuracy class
Standard chart for class C current transformers
Relationshipo between the rated burden and rated terminal voltage of a C class CT is linear.
The relationship of terminal rated voltages between different ratios of a multi-ratio CT is also linear (assuming uniformly distributed secondary winding)
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23. Knee point voltage Knee point Log-log plot Square decades
I.e. (.01,1) – (.1,10)
Tangent 45º line (knee pt)
Exciting current vs exciting voltage plot (C Class CT)
Knee points defined as “the voltage at the point where the tangent to the curve (on log-log axes) is at
45 degrees to the horizontal axis.
- The saturation voltage (VSAT) is graphically found by locating the intersection of the
straight portions of the excitation curve on log-log axes
- Note that neither is the same as the rated voltage, eg, 10C400.
10% increase in voltage gives you 50% increase in excitation current in the saturation zone
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24. Differential measurement difficulties
Three measuring conditions
normal load flow - no differential current
external fault - ideally no differential current
internal fault - high differential current
CT saturation
causes false differential current for external faults
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25. CT saturation
CT core may reach saturation flux density due to a combination of dc offset in the fault current, with possibility remnant flux (20-80%, no way to predict, but can be erased by demagenatizing)
The output current suddenly changes from a proportional signal to zero
DC saturation depends on
system time constant (X/R)- large close to generating stations
secondary burden
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26. CT saturation CT secondary model
a perfect current source (infinite impedance) in parallel with an exciting impedance branch that drives proportional current
Exciting impedance is normally very high
At saturation, exciting impedance drops to a very low value
the CT appears short-circuited
neither delivers nor resists current flow
Time to saturation is important in low impedance bus protection (ie. Saturation doesn’t come instantly)
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27. Issues affecting bus protection selection
Bus arrangement
Fixed
Switchable
Availability and characteristics of current transformers
Performance requirements
Speed
Dependability
Security
Sensitivity (for high impedance grounded system)
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28. Types of bus protection
Differential
Differentially connected overcurrent
Percentage-restrained differential (low Z)
High impedance differential
Partial differential
Zone interlocked scheme
Back up schemes (eg. local BF or remote over-reaching zones)
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29. Differentially connected overcurrent relay51
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30. TOTAL OUT OF BUS MINUS IE
External fault case
Protected Bus =
FAULT LOCATION IE IT
TOTAL OUT OF BUS MINUS IE
TOTAL INTO BUS
TOTAL OUT OF BUS
Relay
Id = IE
IE is the exciting current loss due to slight CT saturation in high external fault level
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31. Internal fault case
Protected Bus I1 I2 I3 I4 Id
Id = I1+ I2 + I3 + I4

32. Overcurrent relay bus differential
Application OK if:
Symmetrical CT secondary current less than 100
Burden less than rated
Typical pickup setting IPU > 10A
Trip delay greater than 3 x primary time constant (L/R)
Protected bus Id
Essentially the CT must have matching characterisitcs and stringent sizing requirement to avoid saturation.
If a 50 speed is desired, then the pick up set point must be above all possible CT measuring errors (less sensitive).
No built-in protection to CT transient behavior (saturation)
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33. Resistor added to relay branch
External fault
Stabilizing resistor Rd
Total primary contribution to external fault IF (out) through saturated ct RS
ZE = 0
Overcurrent Relay Id
IF (in) total from other cts
Resistor reduces current in relay and increases current in RS (secondary and lead)
Increases sensitivity to internal faults
For external faults, ct saturation case, adding resistor to relay path will reduce current through the relay and push more current through the CT secondary, hence relay will become more stabilized for through faults.
- Basically a high impedance bus differential relay
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34. Multiple restraint Percentage Differential (Legacy)
PROTECTED BUS A B C D
RESTRAINT
R,S AND T ARE
COILS
All circuits have 10 Amp contribution
Restraint = 60
Operate = 0 R S 10 10 10 30
Net restraint (0) T
(20) 20 20
(40)
ONLY ONE PHASE
SHOWN
OPERATE COIL
External fault example – 4 circuit connection
This is how the restraint feature was implemented in legacy bus differential relays.
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35. Multiple restraint Percentage Differential (Legacy)
PROTECTED BUS A B C D
RESTRAINT
R,S AND T ARE
COILS
All circuits have 10 Amp contribution
Restraint = 0
Operate = 40 R S 10 10 10 10
Net restraint (0) T
(0) 20 20
(0)
ONLY ONE PHASE
SHOWN 40
OPERATE COIL
Internal fault example – 4 circuit connection
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36. Torque for typical multi-restraint relay
Operating
Torque
Two Restraint
One Restraint
Amperes
The amount of restraint current required to overcome a certain operating torque. Relays with two restraint windings can overcome higher amount of operating torques, improving security on minor CT saturation.
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37. Torque for typical multi-restraint relay
55 A. of restraint overcomes 7 A of operating current with one restraint winding
Operating
Two Restraint
Torque
One Restraint 7 55
Amperes
A certain current restraint can overcome a certain amount of operating torgues. Similar to the modern numerical restraint-operating current characteristics shown in slopes.
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38. Multi-restraint percentage differential
Good sensitivity
Good security
Allows other relays on the same CT core
Different CT ratios with Aux CTs.
Slow compared to high impedance
Number of feeders limited by restraint windings
Each CT is wired to relay (ca be used for other applications: BF, OC backup etc)
Not easily extendable
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39. High impedance bus protection
High resistor (R > 1500 ohm) in series with relay coil and a MOV to prevent over-voltages
High voltage develops for internal faults. lower Voltage will develop on external faults where with ct saturation
Voltage unit must be set higher than the maximum junction point voltage for any external fault
The lowest achievable sensitivity must verified for the application
Proven reliability and very sensitive
Operating times of less than one cycle for internal faults R
Depending on the relay design the stablizing resistor and metal oxide varistor (MOV) may locate in anther hardware, requiring space and external wiring
Some applications connect 86 lockout relay contact in parallel with the relay coil and MOV to further limit the duration of over-voltages during fault.
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40. High impedance bus protection
EXTERNAL FAULT - SECURITY
Setting VR > K*(IF / N ) ( RL +R S)
K = margin factor
IF = Max external fault current
RS = Ct secondary resistance
RL = Lead resistance to junction box
N = ct turns ratio
INTERNAL FAULT - SENSITIVITY
IM I N = (XIE + IR + IV ) N
N = number of circuits
IE = Ct exciting current at VR
IR = resistor current at VR
IV = Varistor current at VR
R > 1500 W
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41. High impedance bus protection
Criteria to be met
Objective: Keep VR and Imin low
Ct secondary loop resistance kept low
Impedance from junction point to relay is of no consequence so good practice to parallel the CTs as close to the CTs as possible.
Theoretically no limitations in the number of parallel CTs but sensitivity reduced.
All cts should have the same ratio and magnetizing characteristics
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42. High impedance bus protection
Criteria to be met
Tapped CT’s may be interconnected with fixed ratio ct’s if attention is given to the autotransformer effect and the overvoltage protective characteristics of the relay
A voltage setting NOT higher than the lowest of all of the relaying accuracy class voltages of the CT’s used in the scheme (400 V for a C400 CT). IE obtained from the excitation character contains large errors (saturated) , causing errors in Imin calculation (higher Imin, over-reaching)
Voltage setting can be lowered by reducing CT lead resistance
Tapped CT ratios has reduced voltage class rating
Ie. The pickup shall not be set > than 800 volt for C800 CTs
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43. Differential comparator (Legacy, Static type)
RADSS/REB103
Developed to lessen restrictions imposed by high impedance
All CT secondary circuits connected via interposing cts
Connection made using a special diode circuit producing rectified incoming, outgoing and differential currents
IIN
IOUT SR
AUX CTs
IDIFF DR
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44. Differential comparator
Single phase connection
IIN is sum of all feeder instantaneous positive values
IOUT is sum of all feeder instantaneous negative values
IDIFF = IIN - IOUT
AUX CTs
Start
IDIFF
VOp
IOUT
VRes
Trip
IIN
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45. Differential comparator
Internal X L1 L2 L3 L4 X
External
External Internal
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46. Differential comparator
Single phase connection
IDIFF is typically small (normally 0) for external fault and restraint voltage, VRes, is greater than operating voltage, VOp.
IDIFF is typically large for an internal fault and operating voltage, VOp, is greater than restraining voltage, VRes. This produces voltage across trip relay.
AUX CTs
Trip / LO
Start
Start
IDIFF
VOp
Trip
IOUT
VRes
Trip
IIN
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47. Differential comparator
All measurement decisions based on three quantities
IDIFF - difference of input current and output current (IDIFF = IIN - IOUT)
IIN - total input current
S - % differential setting
IDIFF > S x IIN
e.g. for setting S=50%, differential current  50% of incoming current before operation
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48. Differential comparator
Advantages over high impedance differential
Lower ct requirements
Allows much higher ct loop resistances
Accommodate different CT ratios / auxiliary CTs
Fast operating times for internal faults
Detects internal ms
Before ct saturation
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49. Numerical differential comparator
Analog input currents are instantaneously sampled and quantized to numerical number
Similar technique to legacy differential comparator, but with measured sampled data
Secondary circuit loop resistance no longer a critical factor
Critical factor is time available to make the measurement, i.e. time to saturation. (only 3ms required to properly restrain for heavy external faults)
Algorithms for Ct saturation Detection and CT state supervision
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50. Differential comparator
Characteristic can be shown in a typical operate-restraint chart
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51. Quick operation for internal fault
REB 670 detects that I_IN goes up while I_OUT goes down at the beginning of the internal fault and enables fast tripping
When ID>Diff Operation Level
trip is issued
Small i: samples
Capital I: Phasors
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52. Proper & secure restrain during external fault
REB 670 detects this short interval when i_in=i_out (after every fault current zero crossing) and restrain properly during external fault
REB 670 detects that I_IN=I_OUT at the beginning of the external fault
Iin = Iout for a short duration before CT saturation, eg. CT saturation detection. The relay restrains properly.
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53. fulfilled & REB is blocked
Fast open CT algorithm
REB 670 detects that I_IN doesn't change while I_OUT goes down when some of the CTs is open/short circuited
ID>Open CT Level
second condition
fulfilled & REB is blocked
Diff Operation Level
Must be set to higher value than Open CT Level
The relay notices the trend and locus of these points and then makes the decision. The relay sees normal load conditions for 500msec and then sees a sudden drop in the current.
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54. Other Features in Modern Numerical Bus IEDs
Each device capable of connecting multiple bays (eg. CTs) in 3- ph or 1-ph design
Multiple differential zones, dynamic bay switching, zone interconnection, and check zone logics
External fault/CT saturation detection, open CT detection algorithms
Blind zone protection (see next 2 slides)
End zone protection (see next 2 slides)
Backup protection (eg. 50/51, 50BF) for each connected bay
Modern substation automation communication (DNP 3.0, IEC61850)
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55. Blind zone detection Blind zone between live tank CT and breaker
A fault in the blind zone makes operation in ZA unnecessary (tie breaker normally open)
ZB cannot detect the fault
Solution: connect BKR NC (open) status to the bay to remove this CT from ZA, ZB (software)
Remove CT dynamically can force operation of ZB
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56. End Zone Protection red=measuring, blue= tripping zone
CTs are used for both feeder and bus protection measurement (live tank CTs)
Regions not overlapped by both red and blue boundaries are blind zones
Bus Under-trip for 3.
Inst. OC enabled after bkr is opened to send DTT (faster)
BF to trigger DTT (slower)
Feeder under-trip for 4.
disconnect CT after bkr is opened to clear the bus (faster)
BF to clear the bus for 4 (delayed)
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57. Other distribution (MV) bus protection methods
Partial differential
Blocking on feeder fault
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58. Partial differential
Seen more in a main-tie-main bus configuration where you can connect the incoming source and the tie in a partial differential circuit to exclude faults in the adjacent bus in case the two buses are to be joined. In that way, each incoming transformer protection only have to coordinate with local bus feeders as if the tie is open. Also, saving a tie relay.
In this case, multiple sources are bound together and must coordinate with downstream protection.
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59. Blocking scheme
For MV or LV systems equipped with dedicated arc flash protection or arc flash not a concern (eg. GIS), no dedicated bus differential is really needed. This type of blocking scheme can provide fast bus bar protection, saving the cost of a dedicated differential relay..
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60. Conventional Blocking Scheme
Zone of protection
T+100 ms
100 ms
I>
I>>
blocking
Traditional busbar protection based on upstream blocking
Dedicated hard-wire signal paths needed
Signal path delay needs to be considered, input and output delay + auxiliary relays
Changes in the protection scheme may require re- wiring
Typical needed delay in incoming relay is over 100 ms
Delay setting with inst. O/C protection
(conventional approach)
Safety marginal, e.g. delay in operation due to CT saturation.
20…40 ms
O/C protection start delay + output relay’s delay
<40 ms
Start delay with receiving relay + retarding time for the blocking signal *)
ALL TOGETHER
100…120 ms
Now we will shortly go though one example of using GOOSE in one protection schema
Blocking based busbar protection
Outgoing feeders sends over copper-wire a blocking signal to incoming source protection if there is fault on the line
A certain delay introduced due to the communication latency and IED processing delays between IEDs

61. Blocking Scheme with IEC-61850 GOOSE
Yes I am!
I’ll block the
Inst. O/C! Block-PHIPTOC!
Delay setting with inst. O/C protection
(REF615 GOOSE approach)
Safety marginal, e.g. delay in operation due to CT saturation.
20…40 ms
O/C protection start delay
20 ms
Retardation time of inst. O/C stage blocking
5 ms
GOOSE delay (Type 1A, Class P1)
<10 ms
ALL TOGETHER
55…75 ms
IEC
Who is interested? PHLPTOC-start!
When same schema is executed with GOOSE blocking signal is sent over the Ethernet based station bus to incoming feeder
Faster communication between IEDs due to the missing I/O output and input delays
More protection scheme examples are shown in 615 series presentations

62. Bus protection comparison chart
COST
EASE OF USE
SENSI-
TIVITY
DEPEND ABILITY
SECURITY
FLEXI-
BILITY
SPEED
SIMPLE OVER-CURRENT
LOW
GOOD
POOR
MULTIPLE RESTRAINT
MED
BEST
HIGH IMPEDANCE
FAST
PERCENTAGE RESTRAINED DIFFERENTIAL
HIGH
PARTIAL
DIFFERENTIAL
BLOCKING
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63. Questions? Recommended reading
ANSI C Guide for Protective Relay Applications to Power system Buses
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64. Thank you for your participation
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