Bus Transfer Systems - Bus Transfer Systems - Requirements, Implementation Requirements, Implementation
and Experiencesand Experiences
IEEE – Industry Applications IEEE – Industry Applications Society, PPIC Conference, Society, PPIC Conference, Toronto, Canada: June 2002Toronto, Canada: June 2002
Amit Raje, Anil Raje
Aartech Solonics Ltd., Bhopal, India
Arvind Chaudhary, Jack McCall
Cooper Power Systems, WI, USA
April 7, 2023 2
Bus Transfer Scheme
Process Continuity Transfer of motor loads from Normal
Source to Alternate Source– Contingencies– Unplanned shut-downs and start-ups– Voltage dips
April 7, 2023 3
Critical Applications
Fossil fuel fired boiler Pressurized water reactor nuclear unit Semiconductor manufacturing plants Chemical plants Petrochemical plants Paper mills etc.
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Examples
Thermal Power Station Auxiliaries– Start-up : Station to Unit Transfer
• Planned
– Shut-down : Unit to Station Transfer• Planned
• Class A Trip : Generator trip, Load throw off etc.
• Class B Trip : Turbine and Boiler trips
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GENERATORTRANSFORMERBREAKER
EHV BUS STATIONTRANSFORMERBREAKER
STATIONTRANSFORMER
STATION BOARDBREAKER INCOMER
STATION BOARD
TIE-2 BREAKER(NC)
UNIT AUXILIARIESEG. BOILER FEED PUMPS,FORCED DRAFT ANDINDUCED DRAFT FANS, ETC.
TIE-1BREAKER
BUS TRANSFERSCHEME
UNIT BOARD
GENERATORTRANSFORMER
GENERATOR
UNIT AUXILIARYTRANSFORMER (UAT)
UNIT AUXILIARYTRANSFORMER (UAT)BREAKER
STATIONP.T.
BUSP.T.
UNITP.T.
MOTOR BUS
MM
Fig 1: Thermal Power Station Bus Transfer System Configuration
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Examples (Contd…)
Industrial Process Plant Auxiliaries– Transfer Conditions
• Planned process start-ups/shut-downs
• Under-Voltage/Frequency/Source equipment failure
– Break in process continuity causes :• delayed restarts, material wastage, O&M costs
– Configurations :• Normally Closed Tie Breaker
• Normally Open Tie Breaker
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SOURCE 1BREAKER
INDUSTRIAL PROCESS PLANT AUXILIARIES: MOTORS, PUMPS ETC.
BUS TRANSFERSCHEME
SOURCE 1TRANSFORMER
MOTOR BUS - 1
Fig 2: Industrial Process Plant Bus Transfer Configuration
MM
SOURCE 1
MOTOR BUS - 2
MM
SOURCE 2
SOURCE 2TRANSFORMER
SOURCE 2BREAKER
BUS TIE
MOTORBUS – 1 P.T.
MOTORBUS –2 P.T.
SOURCE 1P.T.
SOURCE 2P.T.
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Bus Transfer Problem
NORMALSOURCEBREAKER
MOTOR BUS
M
Fig 3: A Simplified Bus Transfer Configuration
ALTERNATESOURCEBREAKER
NORMALSOURCE
ALTERNATESOURCE
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Bus Transfer Problem
Momentory Paralleling– Fault During Transfer
• Interrupt ratings of circuit breakers violated
• Withstand ratings of transformers violated
– Phase Difference Monitoring• Power surge
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Bus Transfer Problem (Contd…)
Open Circuit Condition– Spin Down Characteristics
• Normal source integrity prior to the opening of the normal source breaker.
• Stored energy and motor load inertia– High Inertia
• Fans, Reactor coolant pumps
– Low Inertia
• Centrifugal pumps etc.
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Spin down characteristics of an open circuit motor bus
MOTOR BUSVOLTAGE PHASOR(VBUS)
NORMAL SOURCEVOLTAGE PHASOR(VNORMAL)
VOLTAGE COLLAPSEAND PHASE DECAYCHARACTERISTICSOF THE MOTOR BUS
Bus Transfer Problem (Contd…)
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Bus Transfer Problem (Contd…)
• Adjustable Speed Drives– Motoring Mode, Coast Mode, Regenerative Braking Mode
• Motor under-voltage disconnection from the bus
– Re-energization• Most critical task of an open circuit based transfer
– The motor bus residual voltage magnitude.
– The phase angle between the motor bus residual voltage and the alternate source voltage.
– The phase relationship between the oscillating shaft torque and transient electrical air gap torque, all at the time of re-energization.
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Worst Case Analysis
~ Applying twice the rated voltage Inrush current is
– twice the normal motor starting current– Transient
• 6 to 10 times the rated full load current
– Sub-Transient• 9 to 15 times the rated full load current
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Worst Case Analysis (Contd…)
Possible Effects– An improper re-energization can :
• loosen the stator coils
• loosen the rotor bars of the induction motors
• twist a shaft
• Can even rip the machine from its base plate.
• premature motor failure due to fatigue.
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Bus Transfer Method
Parallel (Hot) Transfer– Advantages
• Bump-less transfer
• Ease of application and operator understanding
– Disadvantages• Increase in available fault current
• Transfer not possible in case of :– Steady state voltage/phase difference
– Electrical fault or abnormal conditions
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Fast Transfer Method
Phase difference monitoring– Simultaneous Transfer (1 - 2 cycles)– Sequential Transfer (5 - 10 cycles)
Advantages :– Minimum interruption of power to the bus– Safe, reliable as well as economic in nature– Paralleling of the normal and alternate sources
is avoided.
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In-Phase Transfer Method
Motor Bus Phasor synchronization estimation in an open circuit condition
Based on a 2nd order approximation of Taylor’s expansion to estimate the phase difference between the motor bus and the alternate source at the time of closure.
(sin
) ( )sin
' ( )sin
' ' ( )T Tclo g
T Tclo g
TTclo g
T0
0 0
2
2 0
April 7, 2023 18
In-Phase Transfer Method
In-Phase transfer of motor bus to alternate source
t = T0( T0), ’(T0), ’’(T0)
t = T0 + Tclosing
ALTERNATE SOURCEVOLTAGE PHASOR(VALTERNATE)
MOTOR BUSVOLTAGE PHASOR(VBUS)
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Residual Voltage Transfer
Voltage decay allowed in an Open Circuit condition upto 20 - 25% before re-energization
Load shedding of auxiliaries usually required to avoid overcurrent trip on re-energization
Loss of process continuity possible
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Bus Transfer Standards
ANSI C50.41 (1982) and NEMA MG-1 (1982)– Magnitude of Phasor difference between
motor bus and alternate source in (V/Hz) should be less than 1.33 p.u.
NEMA MG-1 (1987)– Detailed shaft-motor-driven load analysis
required
April 7, 2023 21
Bus Transfer Experiences
Transfer Initiation– Planned Transfers
• Manual
• Remote SCADA Actuation
– Contingency Transfers• Protective Transfers
– Generator Master Trip, Transformer Trip/Fault
• Auto Transfers– Undervoltage, Underfrequency, df/dt Based Transfers
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Thermal Power StationBus Spin Down Characteristics
April 7, 2023 23
Thermal Power StationLoss of Synchronism
April 7, 2023 24
Thermal Power StationFast Bus Transfer
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Continuous Process Industry Bus Spin Down Characteristics
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Continuous Process Industry Fast Bus Transfer with Auto Initiation
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Continuous Process Industry In Phase Transfer with Auto Initiation
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Investigating a Bus Transfer Case
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Breaker Operations: A key to a successful bus transfer.
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Islanded Transfer
Transfer between two asynchronous sources in In-Phase Mode
Islanded Turbine Operation at No Load Islanded Operation of Co-Generation Plant
with Grid Backup thru Islanded Transfer.
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Conclusions
A Bus Transfer Scheme is a critical necessity in various power generation as well as industrial processing scenarios.
Open Circuit based Bus Transfer Schemes are recommended, in the order :– Fast, In Phase, Residual Transfer
Proven Performance and Features of High Speed Motor Bus Transfers