Bus Bar Schemes

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THE ELECTRIC SUB STATION

04/18/23 THE TRANSFORMER BAY 1

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THE ELECTRIC SUB STATION INTRODUCTION

The present electrical power system is a complex interconnection of Generating stations-Transmission systems- Receiving stations- Distribution systems and Load points.

In all the above phases of power flow, the transfer of electrical energy takes place in the electric sub stations.

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INTRODUCTION

Basically an electrical sub station consists of a number of incoming circuits and out going circuits connected to common bus bar systems.

Bus bars are conducting bars to which a number of incoming or out going circuits are connected.

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INTRODUCTIONEach circuit connected to the bus bar will have

certain electrical component such as circuit breakers, isolators, earth switches, current transformers and voltage transformers.

These components are connected in a definite sequence such that a circuit can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short circuits.

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INTRODUCTION

A sub station receives electrical power from generating station through incoming transmission lines and delivers electrical power through the out going transmission lines.

Sub station is an integral part of a power system and is an important link between the generating stations, transmission systems, distribution systems and load points.

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DESIGN CONSIDERATIONS

The sub station is designed with an objective to provide maximum reliability, flexibility, continuity of service and to meet these objectives with the lowest investment costs that satisfy system requirement.

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DESIGN CONSIDERATIONS0

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System requirements include the selection of optimum voltage levels depending on the load requirements and the transmission distances involved. Generally, the generating source will be far away from the load centers. The advantage of capitalizing on low site costs, availability of ample cooling water supply, economical fuel supply and less stringent environmental considerations compel construction of generating source far away from load centers, there by, increasing transmission distances.

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DESIGN CONSIDERATIONS

Hence, to transmit power over long distances the transmission voltage is to be increased and in our country 400 kV is becoming common and higher voltages for transmission is being explored. Many factors such as voltage level, load capacity, site space limitations, transmission line right of way requirement and environmental considerations influence the design of sub stations.

The system requirement must be met with minimum costs as the cost of equipment, labor, land and site treatment is increasing every day.

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THE ELECTRIC SUB STATION BUS LAYOUT AND SWITCHING ARRANGEMENT

Since the major sub station costs are reflected in the power transformers, circuit breakers and disconnecting switches, the bus layout and switching arrangement selected will determine the number of switches and power circuit breakers required.

A number of factors must be considered in the selection of bus layouts and switching arrangements for a sub station to meet system and station requirements.

A sub station must be reliable, economical, safe, and as simple in design as possible.

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THE ELECTRIC SUB STATION SUB STATION LAYOUT AND BUS BAR SCHEMES

The term layout denotes the physical arrangement of various components in the sub station relative to one another. The layout is significant as it influences the operation, maintenance, cost and protection of the sub station. These aspects are considered while designing the sub station layout.

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With the given number of incoming lines, out going lines, transformers, etc., the sub station can be designed in several alternative ways.

The physical arrangement of the equipment is called the layout of the sub station. The layout is illustrated by means of single line diagrams.

The design of sub station layout need careful consideration of several aspects such as:

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Switching requirement for normal operation. Switching requirement during abnormal

conditions like short circuits and overloads. Degree of flexibility in operations, simplicity. Freedom from total shutdowns. Maintenance requirements, space for

approaching various equipment for maintenance.

Road/ rail for transporting main and auxiliary equipment.

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SUB STATION LAYOUT AND BUS BAR SCHEMES

Safety of personnel. Protective zones for main and back up

protection. Provision for bye pass facilities and for

extensions, space requirements. Technical requirements such as ratings,

clearances, earth system, lightning protection. Requirement for SCADA and communication. Compatibility for local and ambient condition.

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The choice of bus bar schemes for ac yards depend upon several factors mentioned above. The important bus bar schemes are:

Single bus bar Double bus bar with one breaker per circuit Double bus bar with two breaker per circuit Main and transfer bus Ring bus or Mesh scheme Breaker and a half(1 1/2 breaker) arrangement

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The various schemes are generally compared to emphasize their advantage and limitations.

The basis of comparison is generally the degree of reliability & economic justification.

The degree of reliability is evaluated by determining continuity of service under anticipated operating conditions and possible faults.

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VARIOUS BUS BAR SCHEMES

SINGLE BUS BAR SCHEME

BUS

ISOLATOR

BREAKERCURRENT TRANSFORMER

POWER TRANSFORMER

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THE ELECTRIC SUB STATION SINGLE BUS BAR SCHEME

Advantage –Lowest cost

Disadvantage:

Maintenance without interruption of supply is not possible.

Sub station can not be extended without completely de-energizing the sub station

Can be used only where loads can be interrupted or have other supply arrangements. Least flexibility.

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THE ELECTRIC SUB STATION VARIOUS BUS BAR SCHEMES

SINGLE BUS BAR SCHEME WITH BUS SECTIONALISER

BUS SECTION-2

ISOLATOR

BREAKERCURRENT

TRANSFORMER

POWER TRANSFORMER

BUS SECTION-1

SECTIONALISER

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THE ELECTRIC SUB STATION SINGLE BUS BAR SCHEME WITH BUS SECTIONALISER

Sectionalizing the single bus improves slightly the reliability if the incoming and out going circuits are distributed evenly on both the sections.

Where double feed is provided for any single load it is preferable to have one circuit from each section.

In this arrangement each section behaves as a separate bus bar and any outage can be confined to one section of the bus bar.

Only the faulty section will be tripped by bus differential protection.

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THE ELECTRIC SUB STATION DOUBLE BUS BAR WITH ONE BREAKER PER CIRCUIT

BUS-1

BUS-2

BUS COUPLERBREAKER

POWER TRANSFORMER

BREAKER

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THE ELECTRIC SUB STATION DOUBLE BUS BAR WITH ONE BREAKER PER

CIRCUIT This arrangement has the following advantages: Each load may be fed from either bus. Operational flexibility may be increased by grouping the

incoming and out going feeders in separate groups. Either bus bar can be taken out for maintenance. Bus coupler helps in ‘on load change over ‘from one bus

to the other. Adopted where load and continuity justify additional

cost. A major disadvantage is that the breaker can not be

taken out for maintenance without interrupting supply to the concerned circuit.

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THE ELECTRIC SUB STATION DOUBLE BUS BAR WITH ONE BREAKER PER

CIRCUIT

Bus protection scheme may cause loss of sub station when it operates if all circuits are connected to that bus.

High exposure to bus faults. Line breaker failure takes all circuits connected

to that bus out of service. Bus tie breaker failure takes the entire sub

station out of service.

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DOUBLE MAIN BUS & CB BYPASS ISOLATOR SYSTEM

FEEDER1

BUS-2BUS-2

BUS-1BUS-1

T/F-1

BU

S C

OU

PL

ER

FEEDER2

T/F-2

FEEDER3 FEEDER4

BAY1 BAY2

BAY3

BAY4

BAY5

BAY6 BAY7

FOR ANY CB PROBLEM OR FOR PREVENTIVE MAINTANENCE, SUCH FEEDER CAN BE SHIFTED TO ANOTHER BUS AND THE BYPASS ISOLATOR IS CLOSED, THEN PROTECTION IS TRANSFERRED TO BUS COUPLER AND THE FAULTY CB CAN BE ISOLATED.

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MAIN AND TRANSFER BUSMAIN BUS

TRANSFER BUS

BREAKERTIE BREAKER

LINE LINE

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MAIN AND TRANSFER BUS This is an alternative to double bus single

breaker arrangement which provided for change over to either bus for carrying out maintenance on other bus. But it provided no facility for breaker maintenance without interrupting power supply to the concerned circuit.

The main and transfer bus works the other way round.

This arrangement provides facility for carrying out breaker maintenance but does not permit bus maintenance.

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THE ELECTRIC SUB STATION MAIN AND TRANSFER BUS

Any breaker can be taken out of service for maintenance.

Wherever maintenance is required on any breaker, the circuit is changed over to the transfer bus and controlled through the bus coupler breaker.

Potential devices may be used on the main bus for relaying.

The cost is increased due to use of an extra isolator for each circuit and providing interlock for bus coupler and circuit isolators.

Relaying sensitivity decreases as the same bus coupler is used to energize the concerned circuit for all the circuit breakers whenever they are taken out for maintenance.

Failure of bus or any circuit breaker results in shut down of entire sub station.

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DOUBLE MAIN BUS & TRANSFER BUS SYSTEM

FEEDER1

BUS-2BUS-2

BUS-1BUS-1

FEEDER2

TR

AN

SF

ER

BU

S

CO

UP

LE

R

T/F-1 T/F-2

BU

S C

OU

PL

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TRANSFER BUSTRANSFER BUS

FEEDER3 FEEDER4

BAY1 BAY2

BAY3 BAY4 BAY5

BAY6 BAY7 BAY8

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BUS BAR ARRANGEMENTS

• Double Bus Bar Arrangement with Transfer Bus.

This arrangement provides more

additional flexibility, continuity of Power

Supply, permits periodic maintenance

without total shut down as the two main

buses can be operated independently with

the same redundancy.

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THE ELECTRIC SUB STATION DOUBLE BUS BAR WITH TWO BREAKER

PER CIRCUIT

BUS-1

BUS-2

BREAKER

LINE LINE

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THE ELECTRIC SUB STATION DOUBLE BUS BAR WITH TWO BREAKER PER

CIRCUIT

Each circuit has two dedicated breakers. Has flexibility in permitting feeder circuits to be

connected to either bus. Any breaker can be taken out of service for

maintenance. High reliability. Most expensive. Used only in large generating

stations where security of connection is paramount.

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• RING BUS OR MESH SCHEME

ISOLATOR

BREAKER

LINE

LINE

POWER TRANSFORMER

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THE ELECTRIC SUB STATION RING BUS OR MESH SCHEME In this scheme the breakers are arranged in a ring with

circuits connected between breakers.

There are the same number of circuits as there are breakers.

During normal operation, all breakers are closed. For a circuit fault, two breakers are tripped, and in the event one of the breaker fails to operate to clear the fault, an additional circuit will be tripped by operation of breaker- failure back up relays.

During breaker maintenance, the ring is broken, but all lines remain in service.

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THE ELECTRIC SUB STATION RING BUS OR MESH SCHEME The circuits connected to the ring are arranged so that

sources are alternated with loads.

For an extended circuit outage, the line isolator may be opened and the ring can be closed.

No changes to protective relays are required for any of the various operating conditions or during maintenance.

The ring bus scheme is economical in cost, has good reliability, is safe for operation, is flexible, and is normally considered suitable for important sub stations up to a limit of five circuits.

It is common practice to build major sub stations initially as a ring bus; for more than five outgoing circuits, the ring bus is usually developed to the breaker-and-a-half scheme.

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• One and a Half Breaker Arrangement.

This arrangement provides three circuit

breakers for every two circuits. It gives

high security against loss of supply but

higher cost is involved. Hence this is

provided for important 400/220 KV Sub

Stations.

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THE ELECTRIC SUB STATIONBREAKER -AND -A -HALF SCHEME

BUS-1

BUS-2

ISOLATOR

BREAKER

LINE

TIE BREAKER

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I-CONFIGUARATION

FEEDER2 FEEDER4 FEEDER6 FEEDER8 FEEDER10 FEEDER12


BUS-2BUS-2

BUS-1BUS-1

BA

Y1

BA

Y2

BA

Y3

BA

Y4

BA

Y5

BA

Y6

BA

Y7

BA

Y8

BA

Y9

BA

Y1

0B

AY

11

BA

Y1

2

BA

Y1

3B

AY

14

BA

Y1

5

BA

Y1

6B

AY

17

BA

Y1

8

DIA1 DIA2 DIA3 DIA4 DIA5 DIA6

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D-CONFIGUARATION

BUS-2BUS-2

BUS-1BUS-1



BA

Y1

BAY2

BA

Y3

BA

Y4

BAY5

BA

Y6

BA

Y7

BAY8

BA

Y9

BA

Y1

0

BAY11B

AY

12

BA

Y1

3

BAY14

BA

Y1

5

BA

Y1

6

BAY17

BA

Y1

8

DIA1

DIA2

DIA3

DIA4

DIA5

DIA6

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• BREAKER -AND -A -HALF SCHEME

• The breaker and a half scheme, some times called the three switch scheme, has three breakers in series between the main buses.

• Two circuits are connected between the three breakers, hence the term breaker and a half.

• This pattern is repeated along the main buses so that one and a half breakers are used for each circuit.

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THE ELECTRIC SUB STATION BREAKER -AND -A -HALF SCHEME

Under normal operating conditions all breakers are closed and both buses are energized.

A circuit is tripped by opening the two associated circuit breakers.

Tie breaker failure will trip one additional circuit, but no additional circuit is lost if a line trip involves failure of a bus breaker.

Either bus may be taken out of service at any time with no loss of service.

With sources connected opposite loads, it is possible to operate with both buses out of service.

Breaker maintenance can be done with no loss of service, no relay changes, and simple operation of the breaker isolators.

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THE ELECTRIC SUB STATION• BREAKER -AND -A -HALF SCHEME

• The breaker-and-a-half arrangement is more expensive than other schemes, except the double-breaker-double-bus scheme.

• However, the breaker-and-a-half scheme is superior in flexibility, reliability, and safety.

• Protective relaying and automatic re-closing schemes are more complex than for other schemes and hence costly.

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THANK YOU• ANY QUESTIONS ?

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Documents

Bus Bar Schemes