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.

April 7, 2023 4

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

April 7, 2023 5

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

April 7, 2023 6

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.

April 7, 2023 8

Bus Transfer Problem

NORMALSOURCEBREAKER

MOTOR BUS

M

Fig 3: A Simplified Bus Transfer Configuration

ALTERNATESOURCEBREAKER

NORMALSOURCE

ALTERNATESOURCE

April 7, 2023 9

Bus Transfer Problem

Momentory Paralleling– Fault During Transfer

• Interrupt ratings of circuit breakers violated

• Withstand ratings of transformers violated

– Phase Difference Monitoring• Power surge

April 7, 2023 10

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.

April 7, 2023 11

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…)

April 7, 2023 12

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.

April 7, 2023 13

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

April 7, 2023 14

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.

April 7, 2023 15

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

April 7, 2023 16

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.

April 7, 2023 17

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)

April 7, 2023 19

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

April 7, 2023 20

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

April 7, 2023 22

Thermal Power StationBus Spin Down Characteristics

April 7, 2023 23

Thermal Power StationLoss of Synchronism

April 7, 2023 24

Thermal Power StationFast Bus Transfer

April 7, 2023 25

Continuous Process Industry Bus Spin Down Characteristics

April 7, 2023 26

Continuous Process Industry Fast Bus Transfer with Auto Initiation

April 7, 2023 27

Continuous Process Industry In Phase Transfer with Auto Initiation

April 7, 2023 28

Investigating a Bus Transfer Case

April 7, 2023 29

Breaker Operations: A key to a successful bus transfer.

April 7, 2023 30

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.

April 7, 2023 31

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