Type of Substations a Design Review

Type of substations a design review

Electrical Isolator in Substation

Electrical Isolator or Electrical Isolation Switch

Definition of Isolator Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works. So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power system normally at off load condition.

Types of Electrical Isolators There are different types of isolators available depending upon system requirement such as Double Break Isolator Single Break Isolator Pantograph type Isolator Depending upon the position in power system, the isolators can be categorized as Bus side isolator the isolator is directly connected with main bus Line side isolator the isolator is situated at line side of any feeder Transfer bus side isolator the isolator is directly connected with transfer bus

Constructional features of Double Break Isolators Lets have a discussion on constructional features of Double Break Isolators. These have three stacks of post insulators as shown in the figure. The central post insulator carries a tubular or flat male contact which can be rotated horizontally with rotation of central post insulator. This rod type

contact is also called moving contact. The female type contacts are fixed on the top of the other post insulators which fitted at both sides of the central post insulator. The female contacts are generally in the form of spring loaded figure

contacts. The rotational movement of male contact causes to come itself into female contacts and isolators becomes closed. The rotation of male contact in opposite direction make to it out from female contacts and isolators becomes open. Rotation of the central post insulator is done by a driving lever mechanism at the base of the post insulator and it connected to operating handle (in case of hand operation) or motor (in case of motorized operation) of the isolator through a mechanical tie rod.

Constructional features of Single Break Isolators The contact arm is divided into two parts one carries male contact and other female contact. The contact arm moves due to rotation of the post insulator upon which the contact arms are fitted. Rotation of both post insulators stacks in opposite to each other causes to close the isolator by closing the contact arm. Counter rotation of both post insulators stacks open the contact arm and isolator becomes in off condition. This motorized form of this type of isolators is generally used but emergency hand driven mechanism is also provided.

Earthing Switches Earthing switches are mounted on the base of mainly line side isolator. Earthing switches are normally vertically break switches. Earthing arms (contact arm of earthing switch) are normally aligned horizontally at off condition. during switching on operation, these earthing arms rotate and move to vertical position and make contact with earth female contacts fitted at the top of the post insulator stack of isolator at its outgoing side. The erarthing arms are so interlocked with main isolator moving contacts that it can be closed only when the main contacts of isolator are in open position. Similarly the main isolator contacts can be closed only when the earthing arms are in open position.

Operation of Electrical Isolator As no arc quenching technique is provided in isolator it must be operated when there is no chance current flowing through the circuit. No live circuit should be closed or open by isolator operation. A complete live closed circuit must not be opened by isolator operation and also a live circuit must not be closed and completed by isolator operation to avoid huge arcing in between isolator contacts. That is why isolators must be open after circuit breaker is open and these must be closed before circuit breaker is closed. Isolator can be operated by hand locally as well as by motorized mechanism from remote position. Motorized operation arrangement costs more compared to hand operation; hence decision must be taken before choosing an isolator for system whether hand operated or motor operated economically optimum for the system. For voltages up to 145KV system hand operated isolators are used whereas for higher voltage systems like 245 KV or 420 KV and above motorized isolators are used.

Substation Layout

Electrical Bus System & Substation Layout

Electrical Bus System There are many different electrical bus system schemes available but selection of a particular scheme depends upon the system voltage, position of substation in electrical power system, flexibility needed in system and cost to be expensed.

The main criterias to be considered during selection of one particular Bus Bar Arrangement Scheme among others (i) Simplicity of system (ii) Easy maintenance of different equipments. (iii) Minimizing the outage during maintenance. (iv) Future provision of extension with growth of demand (v) Optimizing the selection of bus bar arrangement scheme so that it gives maximum return from the system. Some very commonly used bus bar arrangement are discussed below

Single Bus System Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as shown.

Advantages of single bus system

This is very simple in design This is very cost effective scheme This is very convenient to operate

Disadvantages of single bus system

One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay. The indoor 11KV switchboards have quite often single bus bar arrangement.

Single Bus System with Bus Sectionalizer Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the figure, interruption of system can be reduced to a good extent.

Advantages of single bus system with bus sectionalizer

If any of the sources is out of system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker.

If one section of the bus bar system is under maintenance, part load of the substation can be fed by energizing the other section of bus bar.

Disadvantages of single bus system with bus sectionalizer

As in the case of single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay. The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated off circuit and which is not possible without total interruption of bus bar. So investment for bus-coupler breaker is required.

Double Bus System In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus. Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the figure.

By closing any of the isolators one can put the feeder to associated bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where feeder is transferred. Lastly after this transfer operation he or she should open the bus coupler breaker.

Advantages of Double Bus System

Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System

The arrangement does not permit breaker maintenance with out interruption.

Double Breaker Bus System

In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. Only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown

in the figure. By closing any of the breakers and its associated isolators, one can put the feeder to respective bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus similar to previous case. But any feeder at any time can be transferred from one bus to other. There is no need of bus coupler as because the operation is done by breakers instead of isolator. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred and then he or she opens the breaker and then isolators associated with the bus from where feeder is transferred.

One and a half Breaker Bus System

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers and these two feeders are coupled by a third breaker which is called tie breaker. Normally all the three breakers are closed and power is fed to both the circuits from two buses which are operated in parallel. The tie breaker acts as coupler for the two feeder circuits. During failure of any feeder breaker, the power is fed through the breaker of the second feeder and tie breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by tie breaker.

Advantages of One and a half Breaker Bus System During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and a half Breaker Bus System This scheme is much expensive due to investment for third breaker.

Main and Transfer Bus System

This is an alternative of double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of circuit breaker and associated isolators at both side of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with feeder is switched off or isolated from system, the feeder can still be fed in this way by transferring it to transfer bus.

Switching operation for transferring a feeder to transfer bus from main bus

without interruption of power

(i) First close the isolators at both side of the bus coupler breaker. (ii) Then close the bypass isolator of the feeder which is to be transferred to transfer bus. (iii) Now energized the transfer bus by closing the bus coupler circuit breaker from remote. (iv) After bus coupler breaker is closed, now the power from main bus flows to the feeder line through its main breaker as well as bus coupler breaker via transfer bus. (v) Now if main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker and hence this breaker will serve the purpose of protection for the feeder. (vi) At last the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system. So it can be concluded that in Main & Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage the scheme is very popular for 33KV and 13KV system.

Double Bus System with Bypass Isolators

This is combination of the double bus system and main and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in double bus system but it provides all the advantages of double bus system. It however requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still this scheme is best for optimum economy of system and it is best optimum choice for 220KV system.

Ring Bus System

The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages. One as it is closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing system. Secondly, during maintenance or any other reason if any one of the circuit breaker in ring loop is switch of reliability of system becomes very poor as because closed loop becomes opened. Since, at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between tripped breaker and open end of the loop.

Electrical Substation

Electrical Power Substation Engineering

Electrical Substation Now days the electrical power demand is increasing very rapidly. For fulfilling these huge power demands the modern time requires creation of bigger and bigger power generating stations. These power generating stations may be hydro electric, thermal or atomic. Depending upon the availability of resources these stations are constructed different places. These places may not be nearer to load centers where the actual consumption of power takes place. So it is necessary to transmit these huge power blocks from generating station to their load centers. Long and high voltage transmission networks are needed for this purpose. Power is generated comparatively in low voltage level. It is economical to transmit power at high voltage level. Distribution of electrical power is done at lower voltage levels as specified by consumers. For maintaining these voltage levels and for providing greater stability a number of transformation and switching stations have to be created in between generating station and consumer ends. These transformation and switching stations are generally known as electrical substations. Depending upon the purposes, the substations may be classified as

Step up Substation Step up substations are associated with generating stations. Generation of power is limited to low voltage levels due to limitations of the rotating alternators. These generating voltages must be stepped up for economical transmission of power over long distance. So there must be a step up substation associated with generating station.

Step down Substation The stepped up voltages must be stepped down at load centers, to different voltage levels for different purposes. Depending upon these purposes the step down substation are further categorized in different sub categories.

Primary Step down Substation

The primary step down sub stations are created nearer to load center along the primary transmission lines. Here primary transmission voltages are stepped down to different suitable voltages for secondary transmission purpose.

Secondary Step down Substation

Along the secondary transmission lines, at load center, the secondary transmission voltages are further stepped down for primary distribution purpose. The stepping down of secondary transmission voltages to primary distribution levels are done at secondary step down substation.

Distribution Substation

Distribution Substation are situated where the primary distribution voltages are stepped down to supply voltages for feeding the actual consumers through a distribution network.

Bulk Supply or Industrial Substation Bulk Supply or Industrial Substation are generally a distribution sub station but they are dedicated for one consumer only. An industrial consumer of large or medium supply group may be designated as bulk supply consumer. Individual step down substation is dedicated to these consumers.

Mining Substation The mining substation are very special type of substation and they need special design construction because of extra precautions for safety needed in the operation of electric supply.

Mobile Substation The mobile Substations are also very special purpose sub station temporarily required for construction purpose. For big construction purpose this Substation fulfils the temporary power requirement during construction work. Depending upon the constructional feature categories of sub station may be divided into following manner

Outdoor type sub station

Outdoor type Substation are constructed in open air. Nearly all 132KV, 220KV, 400KV substation are outdoor type substation. Although now days special GIS (Gas Insulated Sub Station) are constructed for Extra High Voltage system which are generally situated under roof.

Indoor Substation The substations are constructed under roof is called indoor type substation. Generally 11KV and sometime 33KV substation are of this type.

Underground Substation The substation are situated at underground is called underground substation. In congested places where place for constructing distribution substation is difficult to find out, one can go for underground sub station scheme.

Pole mounted Substation Pole mounted substation are mainly distribution substation constructed on two pole, four pole and sometime six or more poles structures. In these type of substation fuse protected distribution transformer are mounted on poles along with isolator switches.

TYPES OF BUS BAR SYSTEM

TYPES OF BUS BAR SYSTEM

1 Single Busbar System

Single busbar system is as shown below in figure

Single Busbar System

a. Merits

1. Low Cost

2. Simple to Operate

3. Simple Protection b. Demerits

1. Fault of bus or any circuit breaker results in shut down of entire substation.

2. Difficult to do any maintenance.

3. Bus cannot be extended without completely deenergizing substations.

c. Remarks

1. Used for distribution substations up to 33kV.

2. Not used for large substations.

3. Sectionalizing increases flexibility.

2 Main & Transfer Bus bar System

Main & Transfer Bus is as shown below in figure

a. Merits

1. Low initial & ultimate cost

2. Any breaker can be taken out of service for maintenance.

3. Potential devices may be used on the main bus.

b. Demerits

1. Requires one extra breaker coupler.

2. Switching is somewhat complex when maintaining a breaker.

3. Fault of bus or any circuit breaker results in shutdown of entire substation.

c. Remarks

1. Used for 110kV substations where cost of duplicate bus bar system is not justified.

3 Double Bus bar Single Breaker system Double Bus Bar with Double Breaker is as shown below in figure

a. Merits

1. High flexibility

2. Half of the feeders connected to each bus

b. Demerits

1. Extra bus-coupler circuit breaker necessary.

2. Bus protection scheme may cause loss of substation when it operates.

3. High exposure to bus fault.

4. Line breaker failure takes all circuits connected to the bus out of service.

5. Bus couplers failure takes entire substation out of service.

c. Remarks

Most widely used for 66kV, 132kv, 220kV and important 11kv, 6.6kV, 3.3kV

Substations.

4 Double Bus bar with Double breaker System

Double Bus Bar with Double breaker system is as shown below in figure

a. Merits

1. Each has two associated breakers

2. Has flexibility in permitting feeder circuits to be connected to any bus

3. Any breaker can be taken out of service for maintenance.

4. High reliability

b. Demerits

1. Most expensive

2. Would lose half of the circuits for breaker fault if circuits are not connected to

both the buses.

c. Remarks

1. Not used for usual EHV substations due to high cost.

2. Used only for very important, high power, EHV substations.

5 Double Main Bus & Transfer Busbar System Double main bus & transfer bus system is as shown below in figure

a. Merits

1. Most flexible in operation

2. Highly reliable

3. Breaker failure on bus side breaker removes only one ckt. From service

4. All switching done with breakers

5. Simple operation, no isolator switching required

6. Either main bus can be taken out of service at any time for maintenance.

7. Bus fault does not remove any feeder from the service

b. Demerits

1. High cost due to three buses

c. Remarks

1. Preferred by some utilities for 400kV and 220kV important substations.

6 ONE & HALF BREAKER SCHEME

a. Merits

1. Flexible operation for breaker maintenance.

2. Any breaker can be removed from maintenance without interruption of load.

3. Requires 1 1/2 breaker per feeder.

4. Each circuit fed by two breakers.

5. All switching by breaker.

6. Selective tripping.

b. Demerits

1. One and half breakers per circuit, hence higher cost

2. Any breaker can be removed from maintenance without interruption of load.

c. Remarks

1. Used for 400kV & 220kV substations.

2. Preferred.

7 RING OR MESH ARRANGEMENT

a. Merits

Bus bars gave some operational flexibility.

b. Demerits

1. If fault occurs during bus maintenance, ring gets separated into two sections.

2. Auto-reclosing and protection complex.

3. Requires VTs on all circuits because there is no definite voltage reference

point.

4. Breaker failure during fault on one circuit causes loss of additional circuit because

of breaker failure.

These VTs may be required in all cases for synchronizing live line or voltage

indication

c. Remarks

Most widely used for very large power stations having large no. of incoming and

outgoing lines and high power transfer.