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2. ABB Protective Relay School Webinar Series
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3. Bus Protection Fundamentals
ABB Protective Relay School Webinar Series
Bus Protection Fundamentals
Jack Chang
July 12, 2016

4. Presenter
Jack Chang is the regional technical manager for ABB Inc. in the Substation Automation Products BU supporting customers in Canada. 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 (now ABB and Quanta Services, respectively) and also as an engineering consultant 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 for internal faults and stays stable for external faults.
A Delayed bus trip or a mis-trip could lead 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 for internal fault and stable for external faults
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
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9. Single bus
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10. Main and transfer bus
With this bus configuration any breaker can be taken out of service for maintenance without disrupting the electrical supply via the transfer bus.
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11. Double bus single breaker
Typically seen in GIS SWGR
Flexible, transfer and balance load to either bus
Easy to schedule maintenance
Lots of feeders, seen mainly in big industrial 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
Each link protected by two circuit breakers. Most expensive but reliable and flexible.
Dedicated bus protection each of two main bus bars
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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
Each link protected by 1.5 breaker so to speak.
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
A overlapping set of CTs across the tie breaker poses no problem to a energized bus.
Bus fault beside the CT and Breaker when the breaker is normally closed : no problem. Fault can be cleared by relays operating in both protection zones
However, when the breaker is already in open position, it would be unnecessary for Blue zone to operate.
There is a solution to avoid over-tripping the blue zone
- 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 zone will operate for this fault and trip all breakers in the zone, including this tie breaker, but Green relay is restrained. Even though the blue zone is cleared, the fault is still persisting.
Local breaker failure protection (blue zone) could be used to initiate back-up tripping if time delay is acceptable.
Very rare in likelihood of occurrence
There is a solution for this type of “under-tripping” scenario.
The correct solution is to “disconnect” the two CTs to both relays when the breaker is open. This will speed up the tripping.
If the breaker is operated normally open, logic can be made to disconnect the CTs to avoid over-tripping (blue) and under-tripping (Green)
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20. Current Transformers: Equivalent circuit of a CT
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21. 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)
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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22. Steady State Performance of CT
VB – Burden voltage
KS – a measure of how close to saturation for a given application. Used in time to saturate calc.
VSM – saturation voltage at 20 time rated secondary current at 10% error (10A exciting current)
Vx – rated CT voltage, voltage rated across the burden with 10% error and 20 times rated CT current without driving the CT into Saturation
VK – CT Knee point voltage is the voltage at the point where the tangent to the curve (on log-log axes) is at 45 degrees to the abscissa

23. 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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24. Standard chart for class C current transformers
ANSI accuracy class
Standard chart for class C current transformers
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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 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
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27. Issues effecting 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. remote over-reaching zones)
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29. Differentially connected over-current relay51
The OC element must be set to overcome any CT Steady State measurement error plus margin and is does not deal very well with CT saturation.
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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 characteristics 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
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33. 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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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 = 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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35. Torque for typical multi-restraint relay
Operating
Torque
Two Restraint
One Restraint
Amperes
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36. 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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37. 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
Not easily extendable
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38. 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 under 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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39. High impedance bus protection
EXTERNAL FAULT - SECURITY
Setting VR > (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
X = number of circuits
IE = Ct exciting current at VR
IR = resistor current at VR
IV = Varistor current at VR
R > 1500 W
Assuming a completely saturated CT B to calculate VR (the operating voltage) and minimum operating current. IE can normally be read off the CT excitation curve with one exception. IV (current through MOV) can normally be ignored for operating voltage < 1000 V.
If CT B is only partially or minutely saturated, then the developed VR = I2 x (RL + RCT) -VE (CT exciting voltage) – , which will be smaller.
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40. 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 with each parallel connection
Setting VR higher that the CT rated voltage (400V for C400 CTs) will produce errors when estimating CT exciting branch current, hence inaccurate sensitivity calculation.
All cts must have the same ratio and magnetizing characteristics
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41. 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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42. 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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43. Differential comparator
Internal X L1 L2 L3 L4 X
External
External Internal
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44. 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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45. 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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46. 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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47. Differential comparator
Characteristic can be shown in a typical operate-restraint chart
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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. 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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50. 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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51. 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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52. 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 logic
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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53. Blind zone detection for bus coupler bays
Blind zone between live tank CT and breaker
Fault strikes in the blind zone
Tie bkr normally closed: ZA operates but Zone B restrains -> the fault is persisting
Tie bkr normally open: ZA operates unnecessarily and ZB restrains
Solution: connect BKR NC (open) status to the bay to remove the tie CT from ZA, ZB (software)
ZA will then restrain and ZB operates properly for the blind zone fault
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54. Blind zone detection for feeder bays
Regions not overlapped by both red and blue boundaries are blind zones
CTs are used for both feeder and bus protection measurement (live tank CTs)
Common in HV 1-1/2 stations
For 3. measuring CT can be removed (BBP) to avoid over-tripping on open pos. Fast OC set up to DTT (open pos)
For 4. measuring CT can be removed (BBP) to force tripping on open pos (feeder protection operates first)
Similar to the previous slide the over-tripping and under-tripping zones are shown together for illustration purposes. I do not reckon anyone would design a bus configuration this way. The measuring and fault clearing boundary will typically be symmetric for a bus system.
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55. Other distribution (MV) bus protection methods
Partial differential
Blocking on feeder fault
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56. Partial differential
Seen more in a main-tie-main bus configuration where you can connect the incoming source CT and the tie CT in a partial differential circuit to isolate the two bus bar. The fault on each bus bar and feeder circuits can be protected selectively. It also reduces the amount of coordination required.
In this case, multiple sources are bounded together and must coordinate with downstream protection. This reduces the number of relays needed and coordination requirement.
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57. Blocking scheme
For MV or LV systems equipped with 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 relatively fast bus bar protection, if necessary.
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58. 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 feeder if there is fault in the line
A certain delay introduced due to the delays between IEDs

59. 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 done 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

60. 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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61. Questions? Recommended reading
ANSI C Guide for Protective Relay Applications to Power system Buses
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62. Thank you for your participation
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